internal/fetch: refactor to get info separately

Reorganize FetchModule into two functions.

First, one calls GetModuleInfo to get some prelimary,
cheap information, including the size of the zip.

Then one calls FetchModule with the result of GetModuleInfo.

This allows us to make decisions based on the zip size
before downloading the entire zip.

For golang/go#41452

Change-Id: Ifbffa5882488e31a72e78670caf69c3d97a80546
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/255738
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>

2 files changed, 29 insertions, 0 deletions

diff --git a/internal/stdlib/stdlib.go b/internal/stdlib/stdlib.go
index 2bfe6c48..6917c46e 100644
--- a/internal/stdlib/stdlib.go
+++ b/internal/stdlib/stdlib.go
@@ -258,6 +258,19 @@ func Directory(version string) string {
 	return "src"
 }
 
+// Approximate size of Zip("v1.15.2").
+const estimatedZipSize = 16 * 1024 * 1024
+
+func ZipInfo(requestedVersion string) (resolvedVersion string, zipSize int64, err error) {
+	defer derrors.Wrap(&err, "stdlib.ZipInfo(%q)", requestedVersion)
+
+	resolvedVersion, err = semanticVersion(requestedVersion)
+	if err != nil {
+		return "", 0, err
+	}
+	return resolvedVersion, estimatedZipSize, nil
+}
+
 // Zip creates a module zip representing the entire Go standard library at the
 // given version and returns a reader to it. It also returns the time of the
 // commit for that version. The zip file is in module form, with each path
diff --git a/internal/stdlib/stdlib_test.go b/internal/stdlib/stdlib_test.go
index 77a42c7c..253c84cd 100644
--- a/internal/stdlib/stdlib_test.go
+++ b/internal/stdlib/stdlib_test.go
@@ -174,6 +174,22 @@ func TestZip(t *testing.T) {
 	}
 }
 
+func TestZipInfo(t *testing.T) {
+	UseTestData = true
+	defer func() { UseTestData = false }()
+
+	gotVersion, gotSize, err := ZipInfo("v1.14.6")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if want := "v1.14.6"; gotVersion != want {
+		t.Errorf("version: got %q, want %q", gotVersion, want)
+	}
+	if want := int64(estimatedZipSize); gotSize != want {
+		t.Errorf("size: got %d, want %d", gotSize, want)
+	}
+}
+
 func TestVersions(t *testing.T) {
 	UseTestData = true
 	defer func() { UseTestData = false }()