Support large large objects.

Fixes #1865.

large_objects.go

@@ -6,6 +6,11 @@ import (
 	"io"
 )
 
+// The PostgreSQL wire protocol has a limit of 1 GB - 1 per message. See definition of
+// PQ_LARGE_MESSAGE_LIMIT in the PostgreSQL source code. To allow for the other data
+// in the message,maxLargeObjectMessageLength should be no larger than 1 GB - 1 KB.
+var maxLargeObjectMessageLength = 1024*1024*1024 - 1024
+
 // LargeObjects is a structure used to access the large objects API. It is only valid within the transaction where it
 // was created.
 //
@@ -68,32 +73,64 @@ type LargeObject struct {
 
 // Write writes p to the large object and returns the number of bytes written and an error if not all of p was written.
 func (o *LargeObject) Write(p []byte) (int, error) {
-	var n int
-	err := o.tx.QueryRow(o.ctx, "select lowrite($1, $2)", o.fd, p).Scan(&n)
-	if err != nil {
-		return n, err
+	nTotal := 0
+	for {
+		expected := len(p) - nTotal
+		if expected == 0 {
+			break
+		} else if expected > maxLargeObjectMessageLength {
+			expected = maxLargeObjectMessageLength
+		}
+
+		var n int
+		err := o.tx.QueryRow(o.ctx, "select lowrite($1, $2)", o.fd, p[nTotal:nTotal+expected]).Scan(&n)
+		if err != nil {
+			return nTotal, err
+		}
+
+		if n < 0 {
+			return nTotal, errors.New("failed to write to large object")
+		}
+
+		nTotal += n
+
+		if n < expected {
+			return nTotal, errors.New("short write to large object")
+		} else if n > expected {
+			return nTotal, errors.New("invalid write to large object")
+		}
 	}
 
-	if n < 0 {
-		return 0, errors.New("failed to write to large object")
-	}
-
-	return n, nil
+	return nTotal, nil
 }
 
 // Read reads up to len(p) bytes into p returning the number of bytes read.
 func (o *LargeObject) Read(p []byte) (int, error) {
-	var res []byte
-	err := o.tx.QueryRow(o.ctx, "select loread($1, $2)", o.fd, len(p)).Scan(&res)
-	copy(p, res)
-	if err != nil {
-		return len(res), err
+	nTotal := 0
+	for {
+		expected := len(p) - nTotal
+		if expected == 0 {
+			break
+		} else if expected > maxLargeObjectMessageLength {
+			expected = maxLargeObjectMessageLength
+		}
+
+		var res []byte
+		err := o.tx.QueryRow(o.ctx, "select loread($1, $2)", o.fd, expected).Scan(&res)
+		copy(p[nTotal:], res)
+		nTotal += len(res)
+		if err != nil {
+			return nTotal, err
+		}
+
+		if len(res) < expected {
+			return nTotal, io.EOF
+		} else if len(res) > expected {
+			return nTotal, errors.New("invalid read of large object")
+		}
 	}
 
-	if len(res) < len(p) {
-		err = io.EOF
-	}
-	return len(res), err
+	return nTotal, nil
 }
 
 // Seek moves the current location pointer to the new location specified by offset.

large_objects_private_test.go

@@ -0,0 +1,20 @@
+package pgx
+
+import (
+	"testing"
+)
+
+// SetMaxLargeObjectMessageLength sets internal maxLargeObjectMessageLength variable
+// to the given length for the duration of the test.
+//
+// Tests using this helper should not use t.Parallel().
+func SetMaxLargeObjectMessageLength(t *testing.T, length int) {
+	t.Helper()
+
+	original := maxLargeObjectMessageLength
+	t.Cleanup(func() {
+		maxLargeObjectMessageLength = original
+	})
+
+	maxLargeObjectMessageLength = length
+}

large_objects_test.go

@@ -13,7 +13,8 @@ import (
 )
 
 func TestLargeObjects(t *testing.T) {
-	t.Parallel()
+	// We use a very short limit to test chunking logic.
+	pgx.SetMaxLargeObjectMessageLength(t, 2)
 
 	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
 	defer cancel()
@@ -34,7 +35,8 @@ func TestLargeObjects(t *testing.T) {
 }
 
 func TestLargeObjectsSimpleProtocol(t *testing.T) {
-	t.Parallel()
+	// We use a very short limit to test chunking logic.
+	pgx.SetMaxLargeObjectMessageLength(t, 2)
 
 	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
 	defer cancel()
@@ -160,7 +162,8 @@ func testLargeObjects(t *testing.T, ctx context.Context, tx pgx.Tx) {
 }
 
 func TestLargeObjectsMultipleTransactions(t *testing.T) {
-	t.Parallel()
+	// We use a very short limit to test chunking logic.
+	pgx.SetMaxLargeObjectMessageLength(t, 2)
 
 	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
 	defer cancel()