cmd/go/internal/modfetch: wrap errors from HTTP response bodies

The Read method on the Body returned from a net/http.Transport does
not wrap errors returned by the underlying io.Reader and returns a
bare io.ErrUnexpectedEOF if the body is shorter than the declared
Content-Length.

Since we can't feasibly add detail on the net/http side without
breaking established users, we must instead add detail on the caller
side. Since the net/http client uses url.Error for most of its own
errors, we use that same error type here.

I have not added a regression test for this change. (While it is
theoretically possible to set up a GOPROXY that returns incorrect
Content-Length headers, the change seems straightforward enough that
it isn't worth the complex test setup.)

Fixes #52727.

Change-Id: Id00b04ae4fd518148106a49188fe169aadbcce2c
Reviewed-on: https://go-review.googlesource.com/c/go/+/406675
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/modfetch/proxy.go

@@ -247,10 +247,17 @@ func (p *proxyRepo) getBytes(path string) ([]byte, error) {
 		return nil, err
 	}
 	defer body.Close()
-	return io.ReadAll(body)
+
+	b, err := io.ReadAll(body)
+	if err != nil {
+		// net/http doesn't add context to Body errors, so add it here.
+		// (See https://go.dev/issue/52727.)
+		return b, &url.Error{Op: "read", URL: pathpkg.Join(p.redactedURL, path), Err: err}
+	}
+	return b, nil
 }
 
-func (p *proxyRepo) getBody(path string) (io.ReadCloser, error) {
+func (p *proxyRepo) getBody(path string) (r io.ReadCloser, err error) {
 	fullPath := pathpkg.Join(p.url.Path, path)
 
 	target := *p.url
@@ -407,14 +414,18 @@ func (p *proxyRepo) Zip(dst io.Writer, version string) error {
 	if err != nil {
 		return p.versionError(version, err)
 	}
-	body, err := p.getBody("@v/" + encVer + ".zip")
+	path := "@v/" + encVer + ".zip"
+	body, err := p.getBody(path)
 	if err != nil {
 		return p.versionError(version, err)
 	}
 	defer body.Close()
 
 	lr := &io.LimitedReader{R: body, N: codehost.MaxZipFile + 1}
 	if _, err := io.Copy(dst, lr); err != nil {
+		// net/http doesn't add context to Body errors, so add it here.
+		// (See https://go.dev/issue/52727.)
+		err = &url.Error{Op: "read", URL: pathpkg.Join(p.redactedURL, path), Err: err}
 		return p.versionError(version, err)
 	}
 	if lr.N <= 0 {