cmd/go: add support for GOPROXY fallback on unexpected errors
URLs in GOPROXY may now be separated with commas (,) or pipes (|). If
a request to a proxy fails with any error (including connection errors
and timeouts) and the proxy URL is followed by a pipe, the go command
will try the request with the next proxy in the list. If the proxy is
followed by a comma, the go command will only try the next proxy if
the error a 404 or 410 HTTP response.
The go command will determine how to connect to the checksum database
using the same logic. Before accessing the checksum database, the go
command sends a request to <proxyURL>/sumdb/<sumdb-name>/supported.
If a proxy responds with 404 or 410, or if any other error occurs and
the proxy URL in GOPROXY is followed by a pipe, the go command will
try the request with the next proxy. If all proxies respond with 404
or 410 or are configured to fall back on errors, the go command will
connect to the checksum database directly.
This CL does not change the default value or meaning of GOPROXY.
Fixes #37367
Change-Id: If53152ec1c3282c67d4909818b666af58884fb2c
Reviewed-on: https://go-review.googlesource.com/c/go/+/223257
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>