go/src/playground/compilerd/storage.go - release.projects.playground - Git at Google

 // Handlers for HTTP requests to save and load playground examples.
 //
 // handlerSave() handles a POST request with bundled playground source code.
 // The bundle is persisted in a database and a unique ID returned.
 // handlerLoad() handles a GET request with an id parameter. It returns the
 // bundle saved under the provided ID, if any.
 // The current implementation uses a MySQL-like SQL database for persistence.

 package main

 import (
 	crand "crypto/rand"
 	"encoding/binary"
 	"encoding/hex"
 	"encoding/json"
 	"flag"
 	"fmt"
 	"log"
 	"net/http"
 	"time"

 	"playground/lib"
 	"playground/lib/lsql"
 )

 var (
 	// Database connection string as specified in
 	// https://github.com/go-sql-driver/mysql/#dsn-data-source-name
 	// Query parameters are not supported.
 	sqlConf = flag.String("sqlconf", "", "The go-sql-driver database connection string. If empty, load and save requests are disabled.")

 	// Testing parameter, use default value for production.
 	// Name of dataset to use. Used as table name prefix (a single SQL database
 	// can contain multiple datasets).
 	dataset = flag.String("dataset", "pg", "Testing: Name of dataset to use (and create if needed, when allowed by setupdb).")

 	// If set, will attempt to create any missing database tables for the given
 	// dataset. Idempotent.
 	setupDB = flag.Bool("setupdb", false, "Whether to create missing database tables for dataset.")
 )

 //////////////////////////////////////////
 // SqlData type definitions

 // === High-level schema ===
 // Each bundle save request generates a unique BundleID.
 // Every BundleID corresponds to exactly one bundle Json, stored as a Unicode
 // text blob. However, a single bundle Json can correspond to an unlimited
 // number of BundleIDs.
 // BundleIDs are mapped to Jsons via the BundleHash, which is a hash value of
 // the Json. The bundleLink table stores records of BundleID (primary key) and
 // BundleHash, as well as creation and last access timestamps for each record.
 // The bundleData table stores records of BundleHash (primary key) and Json.
 // The bundleLink.BundleHash column references bundleData.BundleHash, but can
 // be set to NULL to allow deletion of bundleData entries.
 // The additional layer of indirection allows storing identical bundles more
 // efficiently and makes the bundle ID independent of its contents, allowing
 // implementation of change history, sharing, expiration etc.
 // TODO(ivanpi): Revisit ON DELETE SET NULL when delete functionality is added.
 // === Schema type details ===
 // The BundleID is a 64-character string consisting of URL-friendly characters
 // (alphanumeric, '-', '_', '.', '~'), beginning with an underscore.
 // The timestamps are UTC DATETIME values.
 // The BundleHash is a raw (sequence of 32 bytes) SHA256 hash.
 // The Json is a MEDIUMTEXT (up to 16 MiB) Unicode (utf8mb4) blob.
 // Note: If bundles larger than ~1 MiB are to be stored, the max_allowed_packed
 // SQL connection parameter must be increased.
 // TODO(ivanpi): Normalize the Json (e.g. file order).

 // All SqlData types should be added here, in order of table initialization.
 var dataTypes = []lsql.SqlData{&bundleData{}, &bundleLink{}}

 type bundleData struct {
 	// Raw SHA256 of the bundle contents
 	BundleHash []byte // primary key
 	// The bundle contents
 	Json string
 }

 func (bd *bundleData) TableName() string {
 	return *dataset + "_bundleData"
 }

 func (bd *bundleData) TableDef() *lsql.SqlTable {
 	return lsql.NewSqlTable(bd, []lsql.SqlColumn{
 		{Name: "BundleHash", Type: "BINARY(32)", Null: false},
 		{Name: "Json", Type: "MEDIUMTEXT", Null: false},
 	}, []string{})
 }

 func (bd *bundleData) QueryRefs() []interface{} {
 	return []interface{}{&bd.BundleHash, &bd.Json}
 }

 type bundleLink struct {
 	// 64-byte printable ASCII string
 	BundleID string // primary key
 	// Raw SHA256 of the bundle contents
 	BundleHash []byte // foreign key => bundleData.BundleHash
 	// Link record creation time
 	TCreated time.Time
 	// Link record last access time
 	TLastAccessed time.Time
 }

 func (bl *bundleLink) TableName() string {
 	return *dataset + "_bundleLink"
 }

 func (bl *bundleLink) TableDef() *lsql.SqlTable {
 	return lsql.NewSqlTable(bl, []lsql.SqlColumn{
 		{Name: "BundleID", Type: "CHAR(64) CHARACTER SET ascii", Null: false},
 		{Name: "BundleHash", Type: "BINARY(32)", Null: true},
 		{Name: "TCreated", Type: "DATETIME", Null: false},
 		{Name: "TLastAccessed", Type: "DATETIME", Null: false},
 	}, []string{
 		"FOREIGN KEY (BundleHash) REFERENCES " + (&bundleData{}).TableName() + "(BundleHash) ON DELETE SET NULL",
 	})
 }

 func (bl *bundleLink) QueryRefs() []interface{} {
 	return []interface{}{&bl.BundleID, &bl.BundleHash, &bl.TCreated, &bl.TLastAccessed}
 }

 //////////////////////////////////////////
 // HTTP request handlers

 // GET request that returns the saved bundle for the given id.
 func handlerLoad(w http.ResponseWriter, r *http.Request) {
 	if !handleCORS(w, r) {
 		return
 	}

 	// Check method and read GET parameters.
 	if !checkGetMethod(w, r) {
 		return
 	}
 	bId := r.FormValue("id")
 	if bId == "" {
 		storageError(w, http.StatusBadRequest, "Must specify id to load.")
 		return
 	}

 	if !checkDBInit(w) {
 		return
 	}

 	var bLink bundleLink
 	// Get the entry for the provided id.
 	err := dbhRead.EFetch(bId, &bLink)
 	if err == lsql.ErrNoSuchEntity {
 		storageError(w, http.StatusNotFound, "No data found for provided id.")
 		return
 	} else if err != nil {
 		storageInternalError(w, "Error getting bundleLink for id", bId, ":", err)
 		return
 	}

 	var bData bundleData
 	// Get the bundle data for the hash linked in the entry.
 	// Note: This can fail if the bundle is deleted between fetching bundleLink
 	// and bundleData. However, it is highly unlikely, costly to mitigate (using
 	// a serializable transaction), and unimportant (error 500 instead of 404).
 	err = dbhRead.EFetch(bLink.BundleHash, &bData)
 	if err != nil {
 		storageInternalError(w, "Error getting bundleData for id", bId, ":", err)
 		return
 	}

 	// Update the last access timestamp for the link.
 	// Since we don't use transactions, this might get clobbered by a slightly
 	// older timestamp.
 	for try := 0; try < 3; try++ {
 		bLink.TLastAccessed = time.Now()
 		// Ignore no rows affected. It means either the timestamp is up to date (no
 		// change was needed) or the record had been deleted.
 		if err = dbhSeq.EUpdate(&bLink); err == nil || err == lsql.ErrNoRowsAffected {
 			err = nil
 			break
 		}
 	}
 	if err != nil {
 		storageInternalError(w, "Error updating last access timestamp for id", bId, ":", err)
 		return
 	}

 	storageRespond(w, http.StatusOK, &StorageResponse{
 		Link: bId,
 		Data: bData.Json,
 	})
 	return
 }

 // POST request that saves the body as a new bundle and returns the bundle id.
 func handlerSave(w http.ResponseWriter, r *http.Request) {
 	if !handleCORS(w, r) {
 		return
 	}

 	// Check method and read POST body.
 	requestBody := getPostBody(w, r)
 	if requestBody == nil {
 		return
 	}
 	if len(requestBody) > maxSize {
 		storageError(w, http.StatusBadRequest, "Program too large.")
 		return
 	}

 	if !checkDBInit(w) {
 		return
 	}

 	// TODO(ivanpi): Check if bundle is parseable. Format/lint?

 	bHashFixed := rawHash(requestBody)
 	bHash := bHashFixed[:]

 	randomLink := func() string {
 		h := make([]byte, 16, 16+len(bHash))
 		err := binary.Read(crand.Reader, binary.LittleEndian, h)
 		if err != nil {
 			panic(fmt.Errorf("rng failed: %v", err))
 		}
 		return "_" + stringHash(append(h, bHash...))[1:]
 	}

 	bNewData := bundleData{
 		BundleHash: bHash,
 		Json:       string(requestBody),
 	}
 	bNewLink := bundleLink{
 		BundleID:   randomLink(),
 		BundleHash: bHash,
 	}

 	// TODO(ivanpi): The function is not idempotent, there is a small probability
 	// of making a duplicate entry (if a commit succeeds, but a transient network
 	// issue reports it as failed).
 	err := dbhSeq.RunInTransaction(3, func(txh *lsql.DBHandle) error {
 		// If a bundleLink entry exists for the generated BundleID, regenerate
 		// BundleID and retry. Buying lottery ticket optional.
 		bLinkFound, err := txh.EExists(bNewLink.BundleID, &bNewLink)
 		if err != nil {
 			log.Println("error checking bundleLink existence for id", bNewLink.BundleID, ":", err)
 			return err
 		} else if bLinkFound {
 			log.Println("random generation resulted in duplicate id", bNewLink.BundleID)
 			bNewLink.BundleID = randomLink()
 			return lsql.RetryTransaction
 		}

 		// Check if a bundleData entry exists for this BundleHash.
 		bDataFound, err := txh.EExists(bNewData.BundleHash, &bNewData)
 		if err != nil {
 			log.Println("error checking bundleData existence for hash", hex.EncodeToString(bHash), ":", err)
 			return err
 		} else if !bDataFound {
 			// If not, save the bundleData.
 			err = txh.EInsert(&bNewData)
 			if err != nil {
 				log.Println("error storing bundleData for hash", hex.EncodeToString(bHash), ":", err)
 				return err
 			}
 		}

 		// Save the bundleLink with the generated BundleID referring to the
 		// bundleData. Assign timestamps just before saving.
 		bNewLink.TCreated = time.Now()
 		bNewLink.TLastAccessed = bNewLink.TCreated
 		err = txh.EInsert(&bNewLink)
 		if err != nil {
 			log.Println("error storing bundleLink for id", bNewLink.BundleID, ":", err)
 			return err
 		}

 		return nil
 	})

 	if err == nil {
 		storageRespond(w, http.StatusOK, &StorageResponse{
 			Link: bNewLink.BundleID,
 			Data: bNewData.Json,
 		})
 	} else if err == lsql.ErrTooManyRetries {
 		storageInternalError(w, err)
 	} else {
 		// An informative error message has already been printed.
 		storageInternalError(w)
 	}
 	return
 }

 //////////////////////////////////////////
 // Response handling

 type StorageResponse struct {
 	// Error message. If empty, request was successful.
 	Error string
 	// Bundle ID for the saved/loaded bundle.
 	Link string
 	// Contents of the loaded bundle.
 	Data string
 }

 // Sends response to client. Request handler should exit after this call.
 func storageRespond(w http.ResponseWriter, status int, body *StorageResponse) {
 	bodyJson, _ := json.Marshal(body)
 	w.Header().Add("Content-Type", "application/json")
 	w.Header().Add("Content-Length", fmt.Sprintf("%d", len(bodyJson)))
 	w.WriteHeader(status)
 	w.Write(bodyJson)
 }

 // Sends error response with specified message to client.
 func storageError(w http.ResponseWriter, status int, msg string) {
 	storageRespond(w, status, &StorageResponse{
 		Error: msg,
 	})
 }

 // Logs error internally and sends non-specific error response to client.
 func storageInternalError(w http.ResponseWriter, v ...interface{}) {
 	if len(v) > 0 {
 		log.Println(v...)
 	}
 	storageError(w, http.StatusInternalServerError, "Internal error, please retry.")
 }

 //////////////////////////////////////////
 // SQL database handles

 // Data writes for the schema are complex enough to require transactions with
 // SERIALIZABLE isolation. However, reads do not require SERIALIZABLE. Since
 // database/sql only allows setting transaction isolation per connection,
 // a separate connection with only READ-COMMITTED isolation is used for reads
 // to reduce lock contention and deadlock frequency.

 var (
 	// Database handle with SERIALIZABLE transaction isolation.
 	// Used for read-write transactions.
 	dbhSeq *lsql.DBHandle
 	// Database handle with READ_COMMITTED transaction isolation.
 	// Used for non-transactional reads.
 	dbhRead *lsql.DBHandle
 )

 func newDBHandle(sqlConfig, txIsolation string, dataTypes []lsql.SqlData, setupdb, readonly bool) (*lsql.DBHandle, error) {
 	// Create a database handle.
 	dbc, err := lib.NewDBConn(sqlConfig, txIsolation)
 	if err != nil {
 		return nil, err
 	}
 	dbh := lsql.NewDBHandle(dbc)
 	if setupdb {
 		// Create missing database tables.
 		for _, t := range dataTypes {
 			if err := dbh.CreateTable(t, true, lib.CreateTableSuffix); err != nil {
 				return nil, fmt.Errorf("failed initializing database tables: %v", err)
 			}
 			// TODO(ivanpi): Initialize database with fixed-ID examples?
 		}
 	}
 	// Prepare simple database queries.
 	for _, t := range dataTypes {
 		if err := dbh.RegisterType(t, readonly); err != nil {
 			return nil, fmt.Errorf("failed preparing database queries: %v", err)
 		}
 	}
 	return dbh, nil
 }

 func initDBHandles() error {
 	if *sqlConf == "" {
 		return nil
 	}

 	var err error
 	// If setupDB is set, tables should be initialized only once, on the handle
 	// that is opened first.
 	if dbhSeq, err = newDBHandle(*sqlConf, "SERIALIZABLE", dataTypes, *setupDB, false); err != nil {
 		return err
 	}
 	// The READ-COMMITTED connection is used only for reads, so it is not
 	// necessary to prepare writing statements such as INSERT.
 	if dbhRead, err = newDBHandle(*sqlConf, "READ-COMMITTED", dataTypes, false, true); err != nil {
 		return err
 	}

 	return nil
 }

 func checkDBInit(w http.ResponseWriter) bool {
 	if dbhSeq == nil || dbhRead == nil {
 		storageError(w, http.StatusInternalServerError, "Internal error: cannot connect to database.")
 		return false
 	}

 	return true
 }
	// Handlers for HTTP requests to save and load playground examples.
	//
	// handlerSave() handles a POST request with bundled playground source code.
	// The bundle is persisted in a database and a unique ID returned.
	// handlerLoad() handles a GET request with an id parameter. It returns the
	// bundle saved under the provided ID, if any.
	// The current implementation uses a MySQL-like SQL database for persistence.

	package main

	import (
	crand "crypto/rand"
	"encoding/binary"
	"encoding/hex"
	"encoding/json"
	"flag"
	"fmt"
	"log"
	"net/http"
	"time"

	"playground/lib"
	"playground/lib/lsql"
	)

	var (
	// Database connection string as specified in
	// https://github.com/go-sql-driver/mysql/#dsn-data-source-name
	// Query parameters are not supported.
	sqlConf = flag.String("sqlconf", "", "The go-sql-driver database connection string. If empty, load and save requests are disabled.")

	// Testing parameter, use default value for production.
	// Name of dataset to use. Used as table name prefix (a single SQL database
	// can contain multiple datasets).
	dataset = flag.String("dataset", "pg", "Testing: Name of dataset to use (and create if needed, when allowed by setupdb).")

	// If set, will attempt to create any missing database tables for the given
	// dataset. Idempotent.
	setupDB = flag.Bool("setupdb", false, "Whether to create missing database tables for dataset.")
	)

	//////////////////////////////////////////
	// SqlData type definitions

	// === High-level schema ===
	// Each bundle save request generates a unique BundleID.
	// Every BundleID corresponds to exactly one bundle Json, stored as a Unicode
	// text blob. However, a single bundle Json can correspond to an unlimited
	// number of BundleIDs.
	// BundleIDs are mapped to Jsons via the BundleHash, which is a hash value of
	// the Json. The bundleLink table stores records of BundleID (primary key) and
	// BundleHash, as well as creation and last access timestamps for each record.
	// The bundleData table stores records of BundleHash (primary key) and Json.
	// The bundleLink.BundleHash column references bundleData.BundleHash, but can
	// be set to NULL to allow deletion of bundleData entries.
	// The additional layer of indirection allows storing identical bundles more
	// efficiently and makes the bundle ID independent of its contents, allowing
	// implementation of change history, sharing, expiration etc.
	// TODO(ivanpi): Revisit ON DELETE SET NULL when delete functionality is added.
	// === Schema type details ===
	// The BundleID is a 64-character string consisting of URL-friendly characters
	// (alphanumeric, '-', '_', '.', '~'), beginning with an underscore.
	// The timestamps are UTC DATETIME values.
	// The BundleHash is a raw (sequence of 32 bytes) SHA256 hash.
	// The Json is a MEDIUMTEXT (up to 16 MiB) Unicode (utf8mb4) blob.
	// Note: If bundles larger than ~1 MiB are to be stored, the max_allowed_packed
	// SQL connection parameter must be increased.
	// TODO(ivanpi): Normalize the Json (e.g. file order).

	// All SqlData types should be added here, in order of table initialization.
	var dataTypes = []lsql.SqlData{&bundleData{}, &bundleLink{}}

	type bundleData struct {
	// Raw SHA256 of the bundle contents
	BundleHash []byte // primary key
	// The bundle contents
	Json string
	}

	func (bd *bundleData) TableName() string {
	return *dataset + "_bundleData"
	}

	func (bd bundleData) TableDef() lsql.SqlTable {
	return lsql.NewSqlTable(bd, []lsql.SqlColumn{
	{Name: "BundleHash", Type: "BINARY(32)", Null: false},
	{Name: "Json", Type: "MEDIUMTEXT", Null: false},
	}, []string{})
	}

	func (bd *bundleData) QueryRefs() []interface{} {
	return []interface{}{&bd.BundleHash, &bd.Json}
	}

	type bundleLink struct {
	// 64-byte printable ASCII string
	BundleID string // primary key
	// Raw SHA256 of the bundle contents
	BundleHash []byte // foreign key => bundleData.BundleHash
	// Link record creation time
	TCreated time.Time
	// Link record last access time
	TLastAccessed time.Time
	}

	func (bl *bundleLink) TableName() string {
	return *dataset + "_bundleLink"
	}

	func (bl bundleLink) TableDef() lsql.SqlTable {
	return lsql.NewSqlTable(bl, []lsql.SqlColumn{
	{Name: "BundleID", Type: "CHAR(64) CHARACTER SET ascii", Null: false},
	{Name: "BundleHash", Type: "BINARY(32)", Null: true},
	{Name: "TCreated", Type: "DATETIME", Null: false},
	{Name: "TLastAccessed", Type: "DATETIME", Null: false},
	}, []string{
	"FOREIGN KEY (BundleHash) REFERENCES " + (&bundleData{}).TableName() + "(BundleHash) ON DELETE SET NULL",
	})
	}

	func (bl *bundleLink) QueryRefs() []interface{} {
	return []interface{}{&bl.BundleID, &bl.BundleHash, &bl.TCreated, &bl.TLastAccessed}
	}

	//////////////////////////////////////////
	// HTTP request handlers

	// GET request that returns the saved bundle for the given id.
	func handlerLoad(w http.ResponseWriter, r *http.Request) {
	if !handleCORS(w, r) {
	return
	}

	// Check method and read GET parameters.
	if !checkGetMethod(w, r) {
	return
	}
	bId := r.FormValue("id")
	if bId == "" {
	storageError(w, http.StatusBadRequest, "Must specify id to load.")
	return
	}

	if !checkDBInit(w) {
	return
	}

	var bLink bundleLink
	// Get the entry for the provided id.
	err := dbhRead.EFetch(bId, &bLink)
	if err == lsql.ErrNoSuchEntity {
	storageError(w, http.StatusNotFound, "No data found for provided id.")
	return
	} else if err != nil {
	storageInternalError(w, "Error getting bundleLink for id", bId, ":", err)
	return
	}

	var bData bundleData
	// Get the bundle data for the hash linked in the entry.
	// Note: This can fail if the bundle is deleted between fetching bundleLink
	// and bundleData. However, it is highly unlikely, costly to mitigate (using
	// a serializable transaction), and unimportant (error 500 instead of 404).
	err = dbhRead.EFetch(bLink.BundleHash, &bData)
	if err != nil {
	storageInternalError(w, "Error getting bundleData for id", bId, ":", err)
	return
	}

	// Update the last access timestamp for the link.
	// Since we don't use transactions, this might get clobbered by a slightly
	// older timestamp.
	for try := 0; try < 3; try++ {
	bLink.TLastAccessed = time.Now()
	// Ignore no rows affected. It means either the timestamp is up to date (no
	// change was needed) or the record had been deleted.
	if err = dbhSeq.EUpdate(&bLink); err == nil \|\| err == lsql.ErrNoRowsAffected {
	err = nil
	break
	}
	}
	if err != nil {
	storageInternalError(w, "Error updating last access timestamp for id", bId, ":", err)
	return
	}

	storageRespond(w, http.StatusOK, &StorageResponse{
	Link: bId,
	Data: bData.Json,
	})
	return
	}

	// POST request that saves the body as a new bundle and returns the bundle id.
	func handlerSave(w http.ResponseWriter, r *http.Request) {
	if !handleCORS(w, r) {
	return
	}

	// Check method and read POST body.
	requestBody := getPostBody(w, r)
	if requestBody == nil {
	return
	}
	if len(requestBody) > maxSize {
	storageError(w, http.StatusBadRequest, "Program too large.")
	return
	}

	if !checkDBInit(w) {
	return
	}

	// TODO(ivanpi): Check if bundle is parseable. Format/lint?

	bHashFixed := rawHash(requestBody)
	bHash := bHashFixed[:]

	randomLink := func() string {
	h := make([]byte, 16, 16+len(bHash))
	err := binary.Read(crand.Reader, binary.LittleEndian, h)
	if err != nil {
	panic(fmt.Errorf("rng failed: %v", err))
	}
	return "_" + stringHash(append(h, bHash...))[1:]
	}

	bNewData := bundleData{
	BundleHash: bHash,
	Json: string(requestBody),
	}
	bNewLink := bundleLink{
	BundleID: randomLink(),
	BundleHash: bHash,
	}

	// TODO(ivanpi): The function is not idempotent, there is a small probability
	// of making a duplicate entry (if a commit succeeds, but a transient network
	// issue reports it as failed).
	err := dbhSeq.RunInTransaction(3, func(txh *lsql.DBHandle) error {
	// If a bundleLink entry exists for the generated BundleID, regenerate
	// BundleID and retry. Buying lottery ticket optional.
	bLinkFound, err := txh.EExists(bNewLink.BundleID, &bNewLink)
	if err != nil {
	log.Println("error checking bundleLink existence for id", bNewLink.BundleID, ":", err)
	return err
	} else if bLinkFound {
	log.Println("random generation resulted in duplicate id", bNewLink.BundleID)
	bNewLink.BundleID = randomLink()
	return lsql.RetryTransaction
	}

	// Check if a bundleData entry exists for this BundleHash.
	bDataFound, err := txh.EExists(bNewData.BundleHash, &bNewData)
	if err != nil {
	log.Println("error checking bundleData existence for hash", hex.EncodeToString(bHash), ":", err)
	return err
	} else if !bDataFound {
	// If not, save the bundleData.
	err = txh.EInsert(&bNewData)
	if err != nil {
	log.Println("error storing bundleData for hash", hex.EncodeToString(bHash), ":", err)
	return err
	}
	}

	// Save the bundleLink with the generated BundleID referring to the
	// bundleData. Assign timestamps just before saving.
	bNewLink.TCreated = time.Now()
	bNewLink.TLastAccessed = bNewLink.TCreated
	err = txh.EInsert(&bNewLink)
	if err != nil {
	log.Println("error storing bundleLink for id", bNewLink.BundleID, ":", err)
	return err
	}

	return nil
	})

	if err == nil {
	storageRespond(w, http.StatusOK, &StorageResponse{
	Link: bNewLink.BundleID,
	Data: bNewData.Json,
	})
	} else if err == lsql.ErrTooManyRetries {
	storageInternalError(w, err)
	} else {
	// An informative error message has already been printed.
	storageInternalError(w)
	}
	return
	}

	//////////////////////////////////////////
	// Response handling

	type StorageResponse struct {
	// Error message. If empty, request was successful.
	Error string
	// Bundle ID for the saved/loaded bundle.
	Link string
	// Contents of the loaded bundle.
	Data string
	}

	// Sends response to client. Request handler should exit after this call.
	func storageRespond(w http.ResponseWriter, status int, body *StorageResponse) {
	bodyJson, _ := json.Marshal(body)
	w.Header().Add("Content-Type", "application/json")
	w.Header().Add("Content-Length", fmt.Sprintf("%d", len(bodyJson)))
	w.WriteHeader(status)
	w.Write(bodyJson)
	}

	// Sends error response with specified message to client.
	func storageError(w http.ResponseWriter, status int, msg string) {
	storageRespond(w, status, &StorageResponse{
	Error: msg,
	})
	}

	// Logs error internally and sends non-specific error response to client.
	func storageInternalError(w http.ResponseWriter, v ...interface{}) {
	if len(v) > 0 {
	log.Println(v...)
	}
	storageError(w, http.StatusInternalServerError, "Internal error, please retry.")
	}

	//////////////////////////////////////////
	// SQL database handles

	// Data writes for the schema are complex enough to require transactions with
	// SERIALIZABLE isolation. However, reads do not require SERIALIZABLE. Since
	// database/sql only allows setting transaction isolation per connection,
	// a separate connection with only READ-COMMITTED isolation is used for reads
	// to reduce lock contention and deadlock frequency.

	var (
	// Database handle with SERIALIZABLE transaction isolation.
	// Used for read-write transactions.
	dbhSeq *lsql.DBHandle
	// Database handle with READ_COMMITTED transaction isolation.
	// Used for non-transactional reads.
	dbhRead *lsql.DBHandle
	)

	func newDBHandle(sqlConfig, txIsolation string, dataTypes []lsql.SqlData, setupdb, readonly bool) (*lsql.DBHandle, error) {
	// Create a database handle.
	dbc, err := lib.NewDBConn(sqlConfig, txIsolation)
	if err != nil {
	return nil, err
	}
	dbh := lsql.NewDBHandle(dbc)
	if setupdb {
	// Create missing database tables.
	for _, t := range dataTypes {
	if err := dbh.CreateTable(t, true, lib.CreateTableSuffix); err != nil {
	return nil, fmt.Errorf("failed initializing database tables: %v", err)
	}
	// TODO(ivanpi): Initialize database with fixed-ID examples?
	}
	}
	// Prepare simple database queries.
	for _, t := range dataTypes {
	if err := dbh.RegisterType(t, readonly); err != nil {
	return nil, fmt.Errorf("failed preparing database queries: %v", err)
	}
	}
	return dbh, nil
	}

	func initDBHandles() error {
	if *sqlConf == "" {
	return nil
	}

	var err error
	// If setupDB is set, tables should be initialized only once, on the handle
	// that is opened first.
	if dbhSeq, err = newDBHandle(sqlConf, "SERIALIZABLE", dataTypes, setupDB, false); err != nil {
	return err
	}
	// The READ-COMMITTED connection is used only for reads, so it is not
	// necessary to prepare writing statements such as INSERT.
	if dbhRead, err = newDBHandle(*sqlConf, "READ-COMMITTED", dataTypes, false, true); err != nil {
	return err
	}

	return nil
	}

	func checkDBInit(w http.ResponseWriter) bool {
	if dbhSeq == nil \|\| dbhRead == nil {
	storageError(w, http.StatusInternalServerError, "Internal error: cannot connect to database.")
	return false
	}

	return true
	}