Add readonlySavepoint function

Includes logic to temporarily disable interrupts while rolling back.

Fixes #103

Author: zombiezen

internal/backend/backend.go

@@ -315,14 +315,11 @@ func (s *Server) getBuild(ctx context.Context, req *jsonrpc.Request) (*jsonrpc.R
 	_, isActive := s.activeBuilds[buildID]
 	s.activeBuildsMu.Unlock()
 
-	if err := sqlitex.Execute(conn, "BEGIN DEFERRED TRANSACTION;", nil); err != nil {
-		return nil, err
+	rollback, err := readonlySavepoint(conn)
+	if err != nil {
+		return nil, fmt.Errorf("get build %v: %v", buildID, err)
 	}
-	defer func() {
-		if err := sqlitex.Execute(conn, "ROLLBACK TRANSACTION;", nil); err != nil {
-			log.Errorf(ctx, "Rollback read-only transaction: %v", err)
-		}
-	}()
+	defer rollback()
 
 	resp := &zbstorerpc.Build{
 		ID:      args.BuildID,

internal/backend/backend_store.go

@@ -13,6 +13,7 @@ import (
 	"io/fs"
 	"iter"
 	"os"
+	"runtime"
 	"slices"
 	"strings"
 	"sync"
@@ -1033,3 +1034,79 @@ func unmarshalJSONString(data string, v any) error {
 	}
 	return nil
 }
+
+// readonlySavepoint starts a new SAVEPOINT.
+// The caller is responsible for calling endFn
+// to roll back the SAVEPOINT and remove it from the transaction stack.
+func readonlySavepoint(conn *sqlite.Conn) (rollbackFunc func(), err error) {
+	name := "readonlySavepoint" // safe as names can be reused
+	var pc [3]uintptr
+	if n := runtime.Callers(0, pc[:]); n > 0 {
+		frames := runtime.CallersFrames(pc[:n])
+		if _, more := frames.Next(); more { // runtime.Callers
+			if _, more := frames.Next(); more { // readonlySavepoint
+				frame, _ := frames.Next() // caller we care about
+				if frame.Function != "" {
+					name = frame.Function
+				}
+			}
+		}
+	}
+
+	startedTransaction := conn.AutocommitEnabled()
+	if err := sqlitex.Execute(conn, `SAVEPOINT "`+name+`";`, nil); err != nil {
+		return nil, err
+	}
+	if startedTransaction {
+		rollbackFunc = func() {
+			panicError := recover()
+
+			if conn.AutocommitEnabled() {
+				// Transaction exited by application. Nothing to roll back.
+				if panicError != nil {
+					panic(panicError)
+				}
+				return
+			}
+
+			// Always run ROLLBACK even if the connection has been interrupted.
+			oldDoneChan := conn.SetInterrupt(nil)
+			defer conn.SetInterrupt(oldDoneChan)
+
+			if err := sqlitex.Execute(conn, "ROLLBACK;", nil); err != nil {
+				panic(err.Error())
+			}
+			if panicError != nil {
+				panic(panicError)
+			}
+		}
+	} else {
+		rollbackFunc = func() {
+			panicError := recover()
+
+			if conn.AutocommitEnabled() {
+				// Transaction exited by application. Nothing to roll back.
+				if panicError != nil {
+					panic(panicError)
+				}
+				return
+			}
+
+			// Always run ROLLBACK even if the connection has been interrupted.
+			oldDoneChan := conn.SetInterrupt(nil)
+			defer conn.SetInterrupt(oldDoneChan)
+
+			if err := sqlitex.Execute(conn, `ROLLBACK TO "`+name+`";`, nil); err != nil {
+				panic(err.Error())
+			}
+			if err := sqlitex.Execute(conn, `RELEASE "`+name+`";`, nil); err != nil {
+				panic(err.Error())
+			}
+			if panicError != nil {
+				panic(panicError)
+			}
+		}
+	}
+
+	return rollbackFunc, nil
+}

internal/backend/realize.go

@@ -872,7 +872,7 @@ func (b *builder) do(ctx context.Context, drvPath zbstore.Path, outputNames sets
 	}()
 
 	// Save outputs as store objects.
-	inputs, err := b.inputs(ctx, conn, drvPath)
+	inputs, err := b.inputs(conn, drvPath)
 	if err != nil {
 		return err
 	}
@@ -927,7 +927,7 @@ func (b *builder) do(ctx context.Context, drvPath zbstore.Path, outputNames sets
 }
 
 // inputs computes the closure of all inputs used by the derivation at drvPath.
-func (b *builder) inputs(ctx context.Context, conn *sqlite.Conn, drvPath zbstore.Path) (map[zbstore.Path]sets.Set[equivalenceClass], error) {
+func (b *builder) inputs(conn *sqlite.Conn, drvPath zbstore.Path) (map[zbstore.Path]sets.Set[equivalenceClass], error) {
 	drv := b.derivations[drvPath]
 	if drv == nil {
 		return nil, fmt.Errorf("input closure for %s: unknown derivation", drvPath)
@@ -952,21 +952,11 @@ func (b *builder) inputs(ctx context.Context, conn *sqlite.Conn, drvPath zbstore
 		}
 	}
 
-	startedTransaction := conn.AutocommitEnabled()
-	if err := sqlitex.Execute(conn, "SAVEPOINT inputs;", nil); err != nil {
+	rollback, err := readonlySavepoint(conn)
+	if err != nil {
 		return nil, fmt.Errorf("input closure for %s: %v", drvPath, err)
 	}
-	defer func() {
-		var sql string
-		if startedTransaction {
-			sql = "ROLLBACK TRANSACTION;"
-		} else {
-			sql = "ROLLBACK TRANSACTION TO SAVEPOINT inputs;"
-		}
-		if err := sqlitex.Execute(conn, sql, nil); err != nil {
-			log.Errorf(ctx, "Rollback read-only savepoint: %v", err)
-		}
-	}()
+	defer rollback()
 
 	for _, input := range drv.InputSources.All() {
 		err := closurePaths(conn, pathAndEquivalenceClass{path: input}, func(pe pathAndEquivalenceClass) bool {