encoding/base32, encoding/base64: document Decode to short buffer

Document that if one of the Decode methods in these packages is given
a short buffer, it panics.

Fixes #69024

Change-Id: I1c0e4c74274965c1cfa0422cc8f86af4fefb1d00
Reviewed-on: https://go-review.googlesource.com/c/go/+/607499
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

src/encoding/base32/base32.go

@@ -387,7 +387,8 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 
 // Decode decodes src using the encoding enc. It writes at most
 // [Encoding.DecodedLen](len(src)) bytes to dst and returns the number of bytes
-// written. If src contains invalid base32 data, it will return the
+// written. The caller must ensure that dst is large enough to hold all
+// the decoded data. If src contains invalid base32 data, it will return the
 // number of bytes successfully written and [CorruptInputError].
 // Newline characters (\r and \n) are ignored.
 func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {

src/encoding/base64/base64.go

@@ -508,7 +508,8 @@ func (d *decoder) Read(p []byte) (n int, err error) {
 
 // Decode decodes src using the encoding enc. It writes at most
 // [Encoding.DecodedLen](len(src)) bytes to dst and returns the number of bytes
-// written. If src contains invalid base64 data, it will return the
+// written. The caller must ensure that dst is large enough to hold all
+// the decoded data. If src contains invalid base64 data, it will return the
 // number of bytes successfully written and [CorruptInputError].
 // New line characters (\r and \n) are ignored.
 func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {