2 files changed, 19 insertions, 2 deletions
diff --git a/src/encoding/base64/base64.go b/src/encoding/base64/base64.go
index 802ef14c38..9445cbd4ef 100644
--- a/src/encoding/base64/base64.go
+++ b/src/encoding/base64/base64.go
@@ -411,7 +411,13 @@ func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err err
 // and returns the extended buffer.
 // If the input is malformed, it returns the partially decoded src and an error.
 func (enc *Encoding) AppendDecode(dst, src []byte) ([]byte, error) {
-	n := enc.DecodedLen(len(src))
+	// Compute the output size without padding to avoid over allocating.
+	n := len(src)
+	for n > 0 && rune(src[n-1]) == enc.padChar {
+		n--
+	}
+	n = decodedLen(n, NoPadding)
+
 	dst = slices.Grow(dst, n)
 	n, err := enc.Decode(dst[len(dst):][:n], src)
 	return dst[:len(dst)+n], err
@@ -643,7 +649,11 @@ func NewDecoder(enc *Encoding, r io.Reader) io.Reader {
 // DecodedLen returns the maximum length in bytes of the decoded data
 // corresponding to n bytes of base64-encoded data.
 func (enc *Encoding) DecodedLen(n int) int {
-	if enc.padChar == NoPadding {
+	return decodedLen(n, enc.padChar)
+}
+
+func decodedLen(n int, padChar rune) int {
+	if padChar == NoPadding {
 		// Unpadded data may end with partial block of 2-3 characters.
 		return n/4*3 + n%4*6/8
 	}
diff --git a/src/encoding/base64/base64_test.go b/src/encoding/base64/base64_test.go
index 4d7437b919..6dfdaef1f1 100644
--- a/src/encoding/base64/base64_test.go
+++ b/src/encoding/base64/base64_test.go
@@ -167,6 +167,13 @@ func TestDecode(t *testing.T) {
 			dst, err := tt.enc.AppendDecode([]byte("lead"), []byte(encoded))
 			testEqual(t, "AppendDecode(%q) = error %v, want %v", p.encoded, err, error(nil))
 			testEqual(t, `AppendDecode("lead", %q) = %q, want %q`, p.encoded, string(dst), "lead"+p.decoded)
+
+			dst2, err := tt.enc.AppendDecode(dst[:0:len(p.decoded)], []byte(encoded))
+			testEqual(t, "AppendDecode(%q) = error %v, want %v", p.encoded, err, error(nil))
+			testEqual(t, `AppendDecode("", %q) = %q, want %q`, p.encoded, string(dst2), p.decoded)
+			if len(dst) > 0 && len(dst2) > 0 && &dst[0] != &dst2[0] {
+				t.Errorf("unexpected capacity growth: got %d, want %d", cap(dst2), cap(dst))
+			}
 		}
 	}
 }