crypto: avoid escaping Hash.Sum on generic architectures

For architectures without a specialized implementation (e.g. arm),
the generic implementation allocates because it does:

	var block = blockGeneric

which causes the compiler to give up trying to analyze block
even though it is technically only ever one implementation.
Instead of a variable, declare a function that wraps blockGeneric.

We apply this fix to md5, sha1, and sha256,
while sha512 already had the equivalent change.
We add a test to all hashing packages to ensure no allocations.

Credit goes to Cuong Manh Le for more specifically identifying
the problem and Keith Randal for suggesting a concrete solution.

Fixes #48055

Change-Id: I1a6a2e028038e051c83fd72b10a8bf4d210df57d
Reviewed-on: https://go-review.googlesource.com/c/go/+/346209
Trust: Joe Tsai <joetsai@digital-static.net>
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/crypto/md5/md5_test.go

@@ -211,6 +211,20 @@ func TestLargeHashes(t *testing.T) {
 	}
 }
 
+func TestAllocations(t *testing.T) {
+	in := []byte("hello, world!")
+	out := make([]byte, 0, Size)
+	h := New()
+	n := int(testing.AllocsPerRun(10, func() {
+		h.Reset()
+		h.Write(in)
+		out = h.Sum(out[:0])
+	}))
+	if n > 0 {
+		t.Errorf("allocs = %d, want 0", n)
+	}
+}
+
 var bench = New()
 var buf = make([]byte, 1024*1024*8+1)
 var sum = make([]byte, bench.Size())

src/crypto/md5/md5block_generic.go

@@ -9,4 +9,6 @@ package md5
 
 const haveAsm = false
 
-var block = blockGeneric
+func block(dig *digest, p []byte) {
+	blockGeneric(dig, p)
+}

src/crypto/sha1/sha1_test.go

@@ -210,6 +210,20 @@ func TestLargeHashes(t *testing.T) {
 	}
 }
 
+func TestAllocations(t *testing.T) {
+	in := []byte("hello, world!")
+	out := make([]byte, 0, Size)
+	h := New()
+	n := int(testing.AllocsPerRun(10, func() {
+		h.Reset()
+		h.Write(in)
+		out = h.Sum(out[:0])
+	}))
+	if n > 0 {
+		t.Errorf("allocs = %d, want 0", n)
+	}
+}
+
 var bench = New()
 var buf = make([]byte, 8192)
 

src/crypto/sha1/sha1block_generic.go

@@ -7,4 +7,6 @@
 
 package sha1
 
-var block = blockGeneric
+func block(dig *digest, p []byte) {
+	blockGeneric(dig, p)
+}

src/crypto/sha256/sha256_test.go

@@ -289,6 +289,20 @@ func TestLargeHashes(t *testing.T) {
 	}
 }
 
+func TestAllocations(t *testing.T) {
+	in := []byte("hello, world!")
+	out := make([]byte, 0, Size)
+	h := New()
+	n := int(testing.AllocsPerRun(10, func() {
+		h.Reset()
+		h.Write(in)
+		out = h.Sum(out[:0])
+	}))
+	if n > 0 {
+		t.Errorf("allocs = %d, want 0", n)
+	}
+}
+
 var bench = New()
 var buf = make([]byte, 8192)
 

src/crypto/sha256/sha256block_generic.go

@@ -7,4 +7,6 @@
 
 package sha256
 
-var block = blockGeneric
+func block(dig *digest, p []byte) {
+	blockGeneric(dig, p)
+}

src/crypto/sha512/sha512_test.go

@@ -888,6 +888,20 @@ func TestLargeHashes(t *testing.T) {
 	}
 }
 
+func TestAllocations(t *testing.T) {
+	in := []byte("hello, world!")
+	out := make([]byte, 0, Size)
+	h := New()
+	n := int(testing.AllocsPerRun(10, func() {
+		h.Reset()
+		h.Write(in)
+		out = h.Sum(out[:0])
+	}))
+	if n > 0 {
+		t.Errorf("allocs = %d, want 0", n)
+	}
+}
+
 var bench = New()
 var buf = make([]byte, 8192)