[go-marshal] Update API

- All Marshal* and Unmarshal* methods now require buffers to be correctly sized
- Only the Copy{In/Out} variants can handle smaller buffers (or address spaces)

PiperOrigin-RevId: 322953881

3 files changed, 34 insertions, 34 deletions

diff --git a/tools/go_marshal/gomarshal/generator.go b/tools/go_marshal/gomarshal/generator.go
index 177013dbb..19bcd4e6a 100644
--- a/tools/go_marshal/gomarshal/generator.go
+++ b/tools/go_marshal/gomarshal/generator.go
@@ -413,13 +413,13 @@ func (g *Generator) Run() error {
 		for _, t := range g.collectMarshallableTypes(a, fsets[i]) {
 			impl := g.generateOne(t, fsets[i])
 			// Collect Marshallable types referenced by the generated code.
-			for ref, _ := range impl.ms {
+			for ref := range impl.ms {
 				ms[ref] = struct{}{}
 			}
 			impls = append(impls, impl)
 			// Collect imports referenced by the generated code and add them to
 			// the list of imports we need to copy to the generated code.
-			for name, _ := range impl.is {
+			for name := range impl.is {
 				if !g.imports.markUsed(name) {
 					panic(fmt.Sprintf("Generated code for '%s' referenced a non-existent import with local name '%s'. Either go-marshal needs to add an import to the generated file, or a package in an input source file has a package name differ from the final component of its path, which go-marshal doesn't know how to detect; use an import alias to work around this limitation.", impl.typeName(), name))
 				}
diff --git a/tools/go_marshal/gomarshal/generator_interfaces_struct.go b/tools/go_marshal/gomarshal/generator_interfaces_struct.go
index 9cd3c9579..4b9cea08a 100644
--- a/tools/go_marshal/gomarshal/generator_interfaces_struct.go
+++ b/tools/go_marshal/gomarshal/generator_interfaces_struct.go
@@ -268,6 +268,10 @@ func (g *interfaceGenerator) emitMarshallableForStruct(st *ast.StructType) {
 	g.emit("// MarshalUnsafe implements marshal.Marshallable.MarshalUnsafe.\n")
 	g.emit("func (%s *%s) MarshalUnsafe(dst []byte) {\n", g.r, g.typeName())
 	g.inIndent(func() {
+		fallback := func() {
+			g.emit("// Type %s doesn't have a packed layout in memory, fallback to MarshalBytes.\n", g.typeName())
+			g.emit("%s.MarshalBytes(dst)\n", g.r)
+		}
 		if thisPacked {
 			g.recordUsedImport("safecopy")
 			g.recordUsedImport("unsafe")
@@ -277,16 +281,13 @@ func (g *interfaceGenerator) emitMarshallableForStruct(st *ast.StructType) {
 					g.emit("safecopy.CopyIn(dst, unsafe.Pointer(%s))\n", g.r)
 				})
 				g.emit("} else {\n")
-				g.inIndent(func() {
-					g.emit("%s.MarshalBytes(dst)\n", g.r)
-				})
+				g.inIndent(fallback)
 				g.emit("}\n")
 			} else {
 				g.emit("safecopy.CopyIn(dst, unsafe.Pointer(%s))\n", g.r)
 			}
 		} else {
-			g.emit("// Type %s doesn't have a packed layout in memory, fallback to MarshalBytes.\n", g.typeName())
-			g.emit("%s.MarshalBytes(dst)\n", g.r)
+			fallback()
 		}
 	})
 	g.emit("}\n\n")
@@ -294,6 +295,10 @@ func (g *interfaceGenerator) emitMarshallableForStruct(st *ast.StructType) {
 	g.emit("// UnmarshalUnsafe implements marshal.Marshallable.UnmarshalUnsafe.\n")
 	g.emit("func (%s *%s) UnmarshalUnsafe(src []byte) {\n", g.r, g.typeName())
 	g.inIndent(func() {
+		fallback := func() {
+			g.emit("// Type %s doesn't have a packed layout in memory, fallback to UnmarshalBytes.\n", g.typeName())
+			g.emit("%s.UnmarshalBytes(src)\n", g.r)
+		}
 		if thisPacked {
 			g.recordUsedImport("safecopy")
 			g.recordUsedImport("unsafe")
@@ -303,16 +308,13 @@ func (g *interfaceGenerator) emitMarshallableForStruct(st *ast.StructType) {
 					g.emit("safecopy.CopyOut(unsafe.Pointer(%s), src)\n", g.r)
 				})
 				g.emit("} else {\n")
-				g.inIndent(func() {
-					g.emit("%s.UnmarshalBytes(src)\n", g.r)
-				})
+				g.inIndent(fallback)
 				g.emit("}\n")
 			} else {
 				g.emit("safecopy.CopyOut(unsafe.Pointer(%s), src)\n", g.r)
 			}
 		} else {
-			g.emit("// Type %s doesn't have a packed layout in memory, fall back to UnmarshalBytes.\n", g.typeName())
-			g.emit("%s.UnmarshalBytes(src)\n", g.r)
+			fallback()
 		}
 	})
 	g.emit("}\n\n")
@@ -463,8 +465,10 @@ func (g *interfaceGenerator) emitMarshallableSliceForStruct(st *ast.StructType,
 			})
 			g.emit("}\n\n")
 
-			g.emit("// Handle any final partial object.\n")
-			g.emit("if length < size*count && length%size != 0 {\n")
+			g.emit("// Handle any final partial object. buf is guaranteed to be long enough for the\n")
+			g.emit("// final element, but may not contain valid data for the entire range. This may\n")
+			g.emit("// result in unmarshalling zero values for some parts of the object.\n")
+			g.emit("if length%size != 0 {\n")
 			g.inIndent(func() {
 				g.emit("idx := limit\n")
 				g.emit("dst[idx].UnmarshalBytes(buf[size*idx:size*(idx+1)])\n")
diff --git a/tools/go_marshal/marshal/marshal.go b/tools/go_marshal/marshal/marshal.go
index cb2166252..85b196f08 100644
--- a/tools/go_marshal/marshal/marshal.go
+++ b/tools/go_marshal/marshal/marshal.go
@@ -58,18 +58,12 @@ type Marshallable interface {
 	// likely make use of the type of these fields).
 	SizeBytes() int
 
-	// MarshalBytes serializes a copy of a type to dst. dst may be smaller than
-	// SizeBytes(), which results in a part of the struct being marshalled. Note
-	// that this may have unexpected results for non-packed types, as implicit
-	// padding needs to be taken into account when reasoning about how much of
-	// the type is serialized.
+	// MarshalBytes serializes a copy of a type to dst.
+	// Precondition: dst must be at least SizeBytes() in length.
 	MarshalBytes(dst []byte)
 
-	// UnmarshalBytes deserializes a type from src. src may be smaller than
-	// SizeBytes(), which results in a partially deserialized struct. Note that
-	// this may have unexpected results for non-packed types, as implicit
-	// padding needs to be taken into account when reasoning about how much of
-	// the type is deserialized.
+	// UnmarshalBytes deserializes a type from src.
+	// Precondition: src must be at least SizeBytes() in length.
 	UnmarshalBytes(src []byte)
 
 	// Packed returns true if the marshalled size of the type is the same as the
@@ -89,8 +83,8 @@ type Marshallable interface {
 	// representation to the dst buffer. This is only safe to do when the type
 	// has no implicit padding, see Marshallable.Packed. When Packed would
 	// return false, MarshalUnsafe should fall back to the safer but slower
-	// MarshalBytes. dst may be smaller than SizeBytes(), see comment for
-	// MarshalBytes for implications.
+	// MarshalBytes.
+	// Precondition: dst must be at least SizeBytes() in length.
 	MarshalUnsafe(dst []byte)
 
 	// UnmarshalUnsafe deserializes a type by directly copying to the underlying
@@ -99,8 +93,8 @@ type Marshallable interface {
 	// This allows much faster unmarshalling of types which have no implicit
 	// padding, see Marshallable.Packed. When Packed would return false,
 	// UnmarshalUnsafe should fall back to the safer but slower unmarshal
-	// mechanism implemented in UnmarshalBytes. src may be smaller than
-	// SizeBytes(), see comment for UnmarshalBytes for implications.
+	// mechanism implemented in UnmarshalBytes.
+	// Precondition: src must be at least SizeBytes() in length.
 	UnmarshalUnsafe(src []byte)
 
 	// CopyIn deserializes a Marshallable type from a task's memory. This may
@@ -149,14 +143,16 @@ type Marshallable interface {
 //
 // Generates four additional functions for marshalling slices of Foos like this:
 //
-// // MarshalUnsafeFooSlice is like Foo.MarshalUnsafe, buf for a []Foo. It's
-// // more efficient that repeatedly calling calling Foo.MarshalUnsafe over a
-// // []Foo in a loop.
+// // MarshalUnsafeFooSlice is like Foo.MarshalUnsafe, buf for a []Foo. It
+// // might be more efficient that repeatedly calling Foo.MarshalUnsafe
+// // over a []Foo in a loop if the type is Packed.
+// // Preconditions: dst must be at least len(src)*Foo.SizeBytes() in length.
 // func MarshalUnsafeFooSlice(src []Foo, dst []byte) (int, error) { ... }
 //
-// // UnmarshalUnsafeFooSlice is like Foo.UnmarshalUnsafe, buf for a []Foo. It's
-// // more efficient that repeatedly calling calling Foo.UnmarshalUnsafe over a
-// // []Foo in a loop.
+// // UnmarshalUnsafeFooSlice is like Foo.UnmarshalUnsafe, buf for a []Foo. It
+// // might be more efficient that repeatedly calling Foo.UnmarshalUnsafe
+// // over a []Foo in a loop if the type is Packed.
+// // Preconditions: src must be at least len(dst)*Foo.SizeBytes() in length.
 // func UnmarshalUnsafeFooSlice(dst []Foo, src []byte) (int, error) { ... }
 //
 // // CopyFooSliceIn copies in a slice of Foo objects from the task's memory.