[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

2 files changed, 18 insertions, 14 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")