Simplify {Un}MarshalUnsafeSlice method signatures.

Earlier this function was returning (int, error) much like the Copy{In/Out}
methods. The returned error was always nil. The returned int was never used.

Instead make it returned the shifted buffer which is more useful.
Updates #6450

PiperOrigin-RevId: 408268327

5 files changed, 42 insertions, 79 deletions

diff --git a/pkg/lisafs/message.go b/pkg/lisafs/message.go
index 0d7c30ce3..c5474d804 100644
--- a/pkg/lisafs/message.go
+++ b/pkg/lisafs/message.go
@@ -307,11 +307,7 @@ func (m *MountResp) MarshalBytes(dst []byte) []byte {
 	dst = m.MaxMessageSize.MarshalUnsafe(dst)
 	numSupported := primitive.Uint16(len(m.SupportedMs))
 	dst = numSupported.MarshalBytes(dst)
-	n, err := MarshalUnsafeMIDSlice(m.SupportedMs, dst)
-	if err != nil {
-		panic(err)
-	}
-	return dst[n:]
+	return MarshalUnsafeMIDSlice(m.SupportedMs, dst)
 }
 
 // UnmarshalBytes implements marshal.Marshallable.UnmarshalBytes.
@@ -320,12 +316,12 @@ func (m *MountResp) UnmarshalBytes(src []byte) []byte {
 	src = m.MaxMessageSize.UnmarshalUnsafe(src)
 	var numSupported primitive.Uint16
 	src = numSupported.UnmarshalBytes(src)
-	m.SupportedMs = make([]MID, numSupported)
-	n, err := UnmarshalUnsafeMIDSlice(m.SupportedMs, src)
-	if err != nil {
-		panic(err)
+	if cap(m.SupportedMs) < int(numSupported) {
+		m.SupportedMs = make([]MID, numSupported)
+	} else {
+		m.SupportedMs = m.SupportedMs[:numSupported]
 	}
-	return src[n:]
+	return UnmarshalUnsafeMIDSlice(m.SupportedMs, src)
 }
 
 // ChannelResp is the response to the create channel request.
@@ -440,11 +436,7 @@ func (w *WalkResp) MarshalBytes(dst []byte) []byte {
 	numInodes := primitive.Uint32(len(w.Inodes))
 	dst = numInodes.MarshalUnsafe(dst)
 
-	n, err := MarshalUnsafeInodeSlice(w.Inodes, dst)
-	if err != nil {
-		panic(err)
-	}
-	return dst[n:]
+	return MarshalUnsafeInodeSlice(w.Inodes, dst)
 }
 
 // UnmarshalBytes implements marshal.Marshallable.UnmarshalBytes.
@@ -459,11 +451,7 @@ func (w *WalkResp) UnmarshalBytes(src []byte) []byte {
 	} else {
 		w.Inodes = w.Inodes[:numInodes]
 	}
-	n, err := UnmarshalUnsafeInodeSlice(w.Inodes, src)
-	if err != nil {
-		panic(err)
-	}
-	return src[n:]
+	return UnmarshalUnsafeInodeSlice(w.Inodes, src)
 }
 
 // WalkStatResp is used to communicate stat results for WalkStat.
@@ -481,11 +469,7 @@ func (w *WalkStatResp) MarshalBytes(dst []byte) []byte {
 	numStats := primitive.Uint32(len(w.Stats))
 	dst = numStats.MarshalUnsafe(dst)
 
-	n, err := linux.MarshalUnsafeStatxSlice(w.Stats, dst)
-	if err != nil {
-		panic(err)
-	}
-	return dst[n:]
+	return linux.MarshalUnsafeStatxSlice(w.Stats, dst)
 }
 
 // UnmarshalBytes implements marshal.Marshallable.UnmarshalBytes.
@@ -498,11 +482,7 @@ func (w *WalkStatResp) UnmarshalBytes(src []byte) []byte {
 	} else {
 		w.Stats = w.Stats[:numStats]
 	}
-	n, err := linux.UnmarshalUnsafeStatxSlice(w.Stats, src)
-	if err != nil {
-		panic(err)
-	}
-	return src[n:]
+	return linux.UnmarshalUnsafeStatxSlice(w.Stats, src)
 }
 
 // OpenAtReq is used to open existing FDs with the specified flags.
@@ -578,11 +558,7 @@ func (f *FdArray) SizeBytes() int {
 func (f *FdArray) MarshalBytes(dst []byte) []byte {
 	arrLen := primitive.Uint32(len(*f))
 	dst = arrLen.MarshalUnsafe(dst)
-	n, err := MarshalUnsafeFDIDSlice(*f, dst)
-	if err != nil {
-		panic(err)
-	}
-	return dst[n:]
+	return MarshalUnsafeFDIDSlice(*f, dst)
 }
 
 // UnmarshalBytes implements marshal.Marshallable.UnmarshalBytes.
@@ -594,11 +570,7 @@ func (f *FdArray) UnmarshalBytes(src []byte) []byte {
 	} else {
 		*f = (*f)[:arrLen]
 	}
-	n, err := UnmarshalUnsafeFDIDSlice(*f, src)
-	if err != nil {
-		panic(err)
-	}
-	return src[n:]
+	return UnmarshalUnsafeFDIDSlice(*f, src)
 }
 
 // CloseReq is used to close(2) FDs.
diff --git a/pkg/lisafs/sample_message.go b/pkg/lisafs/sample_message.go
index 745736b6d..3d4c090e4 100644
--- a/pkg/lisafs/sample_message.go
+++ b/pkg/lisafs/sample_message.go
@@ -55,22 +55,14 @@ func (m *MsgDynamic) SizeBytes() int {
 // MarshalBytes implements marshal.Marshallable.MarshalBytes.
 func (m *MsgDynamic) MarshalBytes(dst []byte) []byte {
 	dst = m.N.MarshalUnsafe(dst)
-	n, err := MarshalUnsafeMsg1Slice(m.Arr, dst)
-	if err != nil {
-		panic(err)
-	}
-	return dst[n:]
+	return MarshalUnsafeMsg1Slice(m.Arr, dst)
 }
 
 // UnmarshalBytes implements marshal.Marshallable.UnmarshalBytes.
 func (m *MsgDynamic) UnmarshalBytes(src []byte) []byte {
 	src = m.N.UnmarshalUnsafe(src)
 	m.Arr = make([]MsgSimple, m.N)
-	n, err := UnmarshalUnsafeMsg1Slice(m.Arr, src)
-	if err != nil {
-		panic(err)
-	}
-	return src[n:]
+	return UnmarshalUnsafeMsg1Slice(m.Arr, src)
 }
 
 // Randomize randomizes the contents of m.
diff --git a/pkg/marshal/marshal.go b/pkg/marshal/marshal.go
index 9e34eae80..a48a5835d 100644
--- a/pkg/marshal/marshal.go
+++ b/pkg/marshal/marshal.go
@@ -150,13 +150,13 @@ type Marshallable interface {
 // // 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) { ... }
+// func MarshalUnsafeFooSlice(src []Foo, dst []byte) []byte { ... }
 //
 // // 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) { ... }
+// func UnmarshalUnsafeFooSlice(dst []Foo, src []byte) []byte { ... }
 //
 // // CopyFooSliceIn copies in a slice of Foo objects from the task's memory.
 // func CopyFooSliceIn(cc marshal.CopyContext, addr hostarch.Addr, dst []Foo) (int, error) { ... }
diff --git a/tools/go_marshal/gomarshal/generator_interfaces_primitive_newtype.go b/tools/go_marshal/gomarshal/generator_interfaces_primitive_newtype.go
index e2387f032..86c2dbdd5 100644
--- a/tools/go_marshal/gomarshal/generator_interfaces_primitive_newtype.go
+++ b/tools/go_marshal/gomarshal/generator_interfaces_primitive_newtype.go
@@ -264,36 +264,36 @@ func (g *interfaceGenerator) emitMarshallableSliceForPrimitiveNewtype(nt *ast.Id
 	g.emit("}\n\n")
 
 	g.emit("// MarshalUnsafe%s is like %s.MarshalUnsafe, but for a []%s.\n", slice.ident, g.typeName(), g.typeName())
-	g.emit("func MarshalUnsafe%s(src []%s, dst []byte) (int, error) {\n", slice.ident, g.typeName())
+	g.emit("func MarshalUnsafe%s(src []%s, dst []byte) []byte {\n", slice.ident, g.typeName())
 	g.inIndent(func() {
 		g.emit("count := len(src)\n")
 		g.emit("if count == 0 {\n")
 		g.inIndent(func() {
-			g.emit("return 0, nil\n")
+			g.emit("return dst\n")
 		})
 		g.emit("}\n")
 		g.emit("size := (*%s)(nil).SizeBytes()\n\n", g.typeName())
 
-		g.emit("dst = dst[:size*count]\n")
-		g.emit("gohacks.Memmove(unsafe.Pointer(&dst[0]), unsafe.Pointer(&src[0]), uintptr(len(dst)))\n")
-		g.emit("return size*count, nil\n")
+		g.emit("buf := dst[:size*count]\n")
+		g.emit("gohacks.Memmove(unsafe.Pointer(&buf[0]), unsafe.Pointer(&src[0]), uintptr(len(buf)))\n")
+		g.emit("return dst[size*count:]\n")
 	})
 	g.emit("}\n\n")
 
 	g.emit("// UnmarshalUnsafe%s is like %s.UnmarshalUnsafe, but for a []%s.\n", slice.ident, g.typeName(), g.typeName())
-	g.emit("func UnmarshalUnsafe%s(dst []%s, src []byte) (int, error) {\n", slice.ident, g.typeName())
+	g.emit("func UnmarshalUnsafe%s(dst []%s, src []byte) []byte {\n", slice.ident, g.typeName())
 	g.inIndent(func() {
 		g.emit("count := len(dst)\n")
 		g.emit("if count == 0 {\n")
 		g.inIndent(func() {
-			g.emit("return 0, nil\n")
+			g.emit("return src\n")
 		})
 		g.emit("}\n")
 		g.emit("size := (*%s)(nil).SizeBytes()\n\n", g.typeName())
 
-		g.emit("src = src[:(size*count)]\n")
-		g.emit("gohacks.Memmove(unsafe.Pointer(&dst[0]), unsafe.Pointer(&src[0]), uintptr(len(src)))\n")
-		g.emit("return size*count, nil\n")
+		g.emit("buf := src[:size*count]\n")
+		g.emit("gohacks.Memmove(unsafe.Pointer(&dst[0]), unsafe.Pointer(&buf[0]), uintptr(len(buf)))\n")
+		g.emit("return src[size*count:]\n")
 	})
 	g.emit("}\n\n")
 }
diff --git a/tools/go_marshal/gomarshal/generator_interfaces_struct.go b/tools/go_marshal/gomarshal/generator_interfaces_struct.go
index 21177d39c..e7e4aef76 100644
--- a/tools/go_marshal/gomarshal/generator_interfaces_struct.go
+++ b/tools/go_marshal/gomarshal/generator_interfaces_struct.go
@@ -545,15 +545,14 @@ func (g *interfaceGenerator) emitMarshallableSliceForStruct(st *ast.StructType,
 	g.emit("}\n\n")
 
 	g.emit("// MarshalUnsafe%s is like %s.MarshalUnsafe, but for a []%s.\n", slice.ident, g.typeName(), g.typeName())
-	g.emit("func MarshalUnsafe%s(src []%s, dst []byte) (int, error) {\n", slice.ident, g.typeName())
+	g.emit("func MarshalUnsafe%s(src []%s, dst []byte) []byte {\n", slice.ident, g.typeName())
 	g.inIndent(func() {
 		g.emit("count := len(src)\n")
 		g.emit("if count == 0 {\n")
 		g.inIndent(func() {
-			g.emit("return 0, nil\n")
+			g.emit("return dst\n")
 		})
-		g.emit("}\n")
-		g.emit("size := (*%s)(nil).SizeBytes()\n\n", g.typeName())
+		g.emit("}\n\n")
 
 		fallback := func() {
 			g.emit("// Type %s doesn't have a packed layout in memory, fall back to MarshalBytes.\n", g.typeName())
@@ -562,7 +561,7 @@ func (g *interfaceGenerator) emitMarshallableSliceForStruct(st *ast.StructType,
 				g.emit("dst = src[idx].MarshalBytes(dst)\n")
 			})
 			g.emit("}\n")
-			g.emit("return size * count, nil\n")
+			g.emit("return dst\n")
 		}
 		if thisPacked {
 			g.recordUsedImport("reflect")
@@ -574,9 +573,10 @@ func (g *interfaceGenerator) emitMarshallableSliceForStruct(st *ast.StructType,
 				g.inIndent(fallback)
 				g.emit("}\n\n")
 			}
-			g.emit("dst = dst[:size*count]\n")
-			g.emit("gohacks.Memmove(unsafe.Pointer(&dst[0]), unsafe.Pointer(&src[0]), uintptr(len(dst)))\n")
-			g.emit("return size * count, nil\n")
+			g.emit("size := (*%s)(nil).SizeBytes()\n", g.typeName())
+			g.emit("buf := dst[:size*count]\n")
+			g.emit("gohacks.Memmove(unsafe.Pointer(&buf[0]), unsafe.Pointer(&src[0]), uintptr(len(buf)))\n")
+			g.emit("return dst[size*count:]\n")
 		} else {
 			fallback()
 		}
@@ -584,15 +584,14 @@ func (g *interfaceGenerator) emitMarshallableSliceForStruct(st *ast.StructType,
 	g.emit("}\n\n")
 
 	g.emit("// UnmarshalUnsafe%s is like %s.UnmarshalUnsafe, but for a []%s.\n", slice.ident, g.typeName(), g.typeName())
-	g.emit("func UnmarshalUnsafe%s(dst []%s, src []byte) (int, error) {\n", slice.ident, g.typeName())
+	g.emit("func UnmarshalUnsafe%s(dst []%s, src []byte) []byte {\n", slice.ident, g.typeName())
 	g.inIndent(func() {
 		g.emit("count := len(dst)\n")
 		g.emit("if count == 0 {\n")
 		g.inIndent(func() {
-			g.emit("return 0, nil\n")
+			g.emit("return src\n")
 		})
-		g.emit("}\n")
-		g.emit("size := (*%s)(nil).SizeBytes()\n\n", g.typeName())
+		g.emit("}\n\n")
 
 		fallback := func() {
 			g.emit("// Type %s doesn't have a packed layout in memory, fall back to UnmarshalBytes.\n", g.typeName())
@@ -601,7 +600,7 @@ func (g *interfaceGenerator) emitMarshallableSliceForStruct(st *ast.StructType,
 				g.emit("src = dst[idx].UnmarshalBytes(src)\n")
 			})
 			g.emit("}\n")
-			g.emit("return size * count, nil\n")
+			g.emit("return src\n")
 		}
 		if thisPacked {
 			g.recordUsedImport("gohacks")
@@ -613,10 +612,10 @@ func (g *interfaceGenerator) emitMarshallableSliceForStruct(st *ast.StructType,
 				g.emit("}\n\n")
 			}
 
-			g.emit("src = src[:(size*count)]\n")
-			g.emit("gohacks.Memmove(unsafe.Pointer(&dst[0]), unsafe.Pointer(&src[0]), uintptr(len(src)))\n")
-
-			g.emit("return count*size, nil\n")
+			g.emit("size := (*%s)(nil).SizeBytes()\n", g.typeName())
+			g.emit("buf := src[:size*count]\n")
+			g.emit("gohacks.Memmove(unsafe.Pointer(&dst[0]), unsafe.Pointer(&buf[0]), uintptr(len(buf)))\n")
+			g.emit("return src[size*count:]\n")
 		} else {
 			fallback()
 		}