ci: fix Vim master build (#907)

The Go version numbers used by the setup-go action do not,
unfortunately, following the versioning scheme used by Go itself. Fix
the version used for the Vim master build.

.github/workflows/vim_master.yml

@@ -10,7 +10,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        go-version: ["1.15beta1"]
+        go-version: ["1.15.0-beta1"]
     runs-on: ${{ matrix.os }}
     env:
       VIM_FLAVOR: vim

internal/cmd/genconfig/genconfig.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"regexp"
 	"sort"
 	"strconv"
 	"strings"
@@ -102,7 +103,7 @@ func writeMaxVersionsScripts() {
 	vs.VimCommand = strconv.Quote(testsetup.VimCommand.String())
 	vs.GvimCommand = strconv.Quote(testsetup.GvimCommand.String())
 	vs.MaxGoVersion = testsetup.GoVersions[len(testsetup.GoVersions)-1]
-	vs.MaxRealGoVersion = strings.TrimPrefix(vs.MaxGoVersion, "go")
+	vs.MaxRealGoVersion = semverGitHubGoVersion(vs.MaxGoVersion)
 
 	goVersionsSet := make(map[string]bool)
 	vimVersionsSet := make(map[string]bool)
@@ -228,6 +229,53 @@ func writeFileFromTmpl(path string, tmpl string, v interface{}) {
 	}
 }
 
+var (
+	betaSuffix = regexp.MustCompile(`beta\d+$`)
+	rcSuffix   = regexp.MustCompile(`rc\d+$`)
+)
+
+// semverGitHubGoVersion is a temporary hack to deal with the fact that the GitHub
+// action setup-go requires a semantic version, despite the fact that Go does
+// not follow semver.
+//
+// More discussion: https://github.com/actions/setup-go/issues/63
+//
+// This function panics in case we don't see a valid Go version, e.g.
+// go1.14beta1rc1
+func semverGitHubGoVersion(v string) string {
+	r := v
+	beta := betaSuffix.FindString(v)
+	if beta != "" {
+		r = strings.TrimSuffix(r, beta)
+	}
+	rc := rcSuffix.FindString(r)
+	if rc != "" {
+		r = strings.TrimSuffix(r, rc)
+	}
+	if beta != "" && rc != "" {
+		panic(fmt.Errorf("invalid Go version %v: had both beta and rc suffixes", v))
+	}
+	if !strings.HasPrefix(r, "go") {
+		panic(fmt.Errorf("invalid Go version %v: missing go prefix", v))
+	}
+	r = strings.TrimPrefix(r, "go")
+	rps := strings.Split(r, ".")
+	if len(rps) == 2 {
+		rps = append(rps, "0")
+	}
+	r = strings.Join(rps, ".")
+	if !semver.IsValid("v" + r) {
+		panic(fmt.Errorf("failed to build valid version from %v", v))
+	}
+	if beta != "" {
+		r = r + "-" + beta
+	}
+	if rc != "" {
+		r = r + "-" + rc
+	}
+	return r
+}
+
 const dockerWorkflowYaml = `# Code generated by genconfig. DO NOT EDIT.
 on:
   push: