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Merge pull request #20561 from mars/postgres-backend-session-locks

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Switch pg backend to session-level advisory locking
pull/20580/head
James Bardin 7 years ago committed by GitHub
parent d72defd044 34fa67c6b0
commit 81c94f7ea7
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
3 changed files with 49 additions and 103 deletions
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138
backend/remote-state/pg/client.go
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@ -1,11 +1,9 @@
package pg
import (
"context"
"crypto/md5"
"database/sql"
"fmt"
"sync"
uuid "github.com/hashicorp/go-uuid"
"github.com/hashicorp/terraform/state"
@ -19,24 +17,12 @@ type RemoteClient struct {
Name string
SchemaName string
// In-flight database transaction. Empty unless Locked.
txn *sql.Tx
txnMux sync.Mutex
info *state.LockInfo
info *state.LockInfo
}
func (c *RemoteClient) Get() (*remote.Payload, error) {
query := `SELECT data FROM %s.%s WHERE name = $1`
var row *sql.Row
// Take exclusive access to the database transaction
c.txnMux.Lock()
defer c.txnMux.Unlock()
// Use the open transaction when present
if c.txn != nil {
row = c.txn.QueryRow(fmt.Sprintf(query, c.SchemaName, statesTableName), c.Name)
} else {
row = c.Client.QueryRow(fmt.Sprintf(query, c.SchemaName, statesTableName), c.Name)
}
row := c.Client.QueryRow(fmt.Sprintf(query, c.SchemaName, statesTableName), c.Name)
var data []byte
err := row.Scan(&data)
switch {
@ -58,16 +44,7 @@ func (c *RemoteClient) Put(data []byte) error {
query := `INSERT INTO %s.%s (name, data) VALUES ($1, $2)
ON CONFLICT (name) DO UPDATE
SET data = $2 WHERE %s.name = $1`
var err error
// Take exclusive access to the database transaction
c.txnMux.Lock()
defer c.txnMux.Unlock()
// Use the open transaction when present
if c.txn != nil {
_, err = c.txn.Exec(fmt.Sprintf(query, c.SchemaName, statesTableName, statesTableName), c.Name, data)
} else {
_, err = c.Client.Exec(fmt.Sprintf(query, c.SchemaName, statesTableName, statesTableName), c.Name, data)
}
_, err := c.Client.Exec(fmt.Sprintf(query, c.SchemaName, statesTableName, statesTableName), c.Name, data)
if err != nil {
return err
}
@ -76,16 +53,7 @@ func (c *RemoteClient) Put(data []byte) error {
func (c *RemoteClient) Delete() error {
query := `DELETE FROM %s.%s WHERE name = $1`
var err error
// Take exclusive access to the database transaction
c.txnMux.Lock()
defer c.txnMux.Unlock()
// Use the open transaction when present
if c.txn != nil {
_, err = c.txn.Exec(fmt.Sprintf(query, c.SchemaName, statesTableName), c.Name)
} else {
_, err = c.Client.Exec(fmt.Sprintf(query, c.SchemaName, statesTableName), c.Name)
}
_, err := c.Client.Exec(fmt.Sprintf(query, c.SchemaName, statesTableName), c.Name)
if err != nil {
return err
}
@ -95,70 +63,62 @@ func (c *RemoteClient) Delete() error {
func (c *RemoteClient) Lock(info *state.LockInfo) (string, error) {
var err error
var lockID string
var txn *sql.Tx
if info.ID == "" {
lockID, err = uuid.GenerateUUID()
if err != nil {
return "", err
}
info.Operation = "client"
info.ID = lockID
}
// Take exclusive access to the database transaction
c.txnMux.Lock()
defer c.txnMux.Unlock()
if c.txn == nil {
// Most strict transaction isolation to prevent cross-talk
// between incomplete state transactions.
txn, err = c.Client.BeginTx(context.Background(), &sql.TxOptions{
Isolation: sql.LevelSerializable,
})
// Local helper function so we can call it multiple places
//
lockUnlock := func(pgLockId string) error {
query := `SELECT pg_advisory_unlock(%s)`
row := c.Client.QueryRow(fmt.Sprintf(query, pgLockId))
var didUnlock []byte
err := row.Scan(&didUnlock)
if err != nil {
return "", &state.LockError{Info: info, Err: err}
return &state.LockError{Info: info, Err: err}
}
c.txn = txn
} else {
return "", &state.LockError{Info: info, Err: fmt.Errorf("Client is already in a locking transaction")}
}
// Do not wait before giving up on a contended lock.
_, err = c.Client.Exec(`SET LOCAL lock_timeout = 0`)
if err != nil {
c.rollback(info)
return "", err
return nil
}
// Try to acquire lock for the existing row.
query := `SELECT pg_try_advisory_xact_lock(%s.id) FROM %s.%s WHERE %s.name = $1`
row := c.txn.QueryRow(fmt.Sprintf(query, statesTableName, c.SchemaName, statesTableName, statesTableName), c.Name)
var didLock []byte
err = row.Scan(&didLock)
// Try to acquire locks for the existing row `id` and the creation lock `-1`.
query := `SELECT %s.id, pg_try_advisory_lock(%s.id), pg_try_advisory_lock(-1) FROM %s.%s WHERE %s.name = $1`
row := c.Client.QueryRow(fmt.Sprintf(query, statesTableName, statesTableName, c.SchemaName, statesTableName, statesTableName), c.Name)
var pgLockId, didLock, didLockForCreate []byte
err = row.Scan(&pgLockId, &didLock, &didLockForCreate)
switch {
case err == sql.ErrNoRows:
// When the row does not yet exist in state, take
// the `-1` lock to create the new row.
innerRow := c.txn.QueryRow(`SELECT pg_try_advisory_xact_lock(-1)`)
// No rows means we're creating the workspace. Take the creation lock.
innerRow := c.Client.QueryRow(`SELECT pg_try_advisory_lock(-1)`)
var innerDidLock []byte
err := innerRow.Scan(&innerDidLock)
if err != nil {
c.rollback(info)
return "", &state.LockError{Info: info, Err: err}
}
if string(innerDidLock) == "false" {
c.rollback(info)
return "", &state.LockError{Info: info, Err: fmt.Errorf("Workspace is already locked: %s", c.Name)}
return "", &state.LockError{Info: info, Err: fmt.Errorf("Already locked for workspace creation: %s", c.Name)}
}
info.Path = "-1"
case err != nil:
c.rollback(info)
return "", &state.LockError{Info: info, Err: err}
case string(didLock) == "false":
c.rollback(info)
// Existing workspace is already locked. Release the attempted creation lock.
lockUnlock("-1")
return "", &state.LockError{Info: info, Err: fmt.Errorf("Workspace is already locked: %s", c.Name)}
case string(didLockForCreate) == "false":
// Someone has the creation lock already. Release the existing workspace because it might not be safe to touch.
lockUnlock(string(pgLockId))
return "", &state.LockError{Info: info, Err: fmt.Errorf("Cannot lock workspace; already locked for workspace creation: %s", c.Name)}
default:
// Existing workspace is now locked. Release the attempted creation lock.
lockUnlock("-1")
info.Path = string(pgLockId)
}
c.info = info
return info.ID, nil
}
@ -168,35 +128,15 @@ func (c *RemoteClient) getLockInfo() (*state.LockInfo, error) {
}
func (c *RemoteClient) Unlock(id string) error {
// Take exclusive access to the database transaction
c.txnMux.Lock()
defer c.txnMux.Unlock()
if c.txn != nil {
err := c.txn.Commit()
if err != nil {
return err
}
c.txn = nil
}
c.info = nil
return nil
}
// This must be called from any code path where the
// transaction would not be committed (unlocked),
// otherwise the transactions will leak and prevent
// the process from exiting cleanly.
//
// Does not use mutex because this will implicitly be
// called from within an already mutex'd scope.
func (c *RemoteClient) rollback(info *state.LockInfo) error {
if c.txn != nil {
err := c.txn.Rollback()
if c.info != nil && c.info.Path != "" {
query := `SELECT pg_advisory_unlock(%s)`
row := c.Client.QueryRow(fmt.Sprintf(query, c.info.Path))
var didUnlock []byte
err := row.Scan(&didUnlock)
if err != nil {
return err
return &state.LockError{Info: c.info, Err: err}
}
c.txn = nil
c.info = nil
}
c.info = nil
return nil
}

12
backend/remote-state/pg/client_test.go
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@ -59,12 +59,18 @@ func TestRemoteLocks(t *testing.T) {
"conn_str": connStr,
"schema_name": schemaName,
})
b := backend.TestBackendConfig(t, New(), config).(*Backend)
s, err := b.StateMgr(backend.DefaultStateName)
b1 := backend.TestBackendConfig(t, New(), config).(*Backend)
s1, err := b1.StateMgr(backend.DefaultStateName)
if err != nil {
t.Fatal(err)
}
b2 := backend.TestBackendConfig(t, New(), config).(*Backend)
s2, err := b2.StateMgr(backend.DefaultStateName)
if err != nil {
t.Fatal(err)
}
remote.TestRemoteLocks(t, s.(*remote.State).Client, s.(*remote.State).Client)
remote.TestRemoteLocks(t, s1.(*remote.State).Client, s2.(*remote.State).Client)
}

2
website/docs/backends/types/pg.html.md
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@ -80,7 +80,7 @@ This backend creates one table **states** in the automatically-managed Postgres
The table is keyed by the [workspace](/docs/state/workspaces.html) name. If workspaces are not in use, the name `default` is used.
Locking is supported using [Postgres advisory locks](https://www.postgresql.org/docs/9.5/explicit-locking.html#ADVISORY-LOCKS). [`force-unlock`](https://www.terraform.io/docs/commands/force-unlock.html) is not supported, because these database-native locks will automatically unlock when the transaction is aborted or the connection fails. To see outstanding locks in a Postgres server, use the [`pg_locks` system view](https://www.postgresql.org/docs/9.5/view-pg-locks.html).
Locking is supported using [Postgres advisory locks](https://www.postgresql.org/docs/9.5/explicit-locking.html#ADVISORY-LOCKS). [`force-unlock`](https://www.terraform.io/docs/commands/force-unlock.html) is not supported, because these database-native locks will automatically unlock when the session is aborted or the connection fails. To see outstanding locks in a Postgres server, use the [`pg_locks` system view](https://www.postgresql.org/docs/9.5/view-pg-locks.html).
The **states** table contains:

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