Add XCADDY_GO_MOD_CACHE_RW env var (caddyserver#102)

Commit 47f9ded is not sufficient alone to
ensure that all go modules are installed with write bit set because go mod
and go get installs modules of other modules with read-only access.

The environment variable sets -modcacherw to both go get and go mod commands
to ensure that the build/pkg/mod directory does not have sub directories
that are read-only for the current user.

README.md

@@ -158,6 +158,7 @@ Because the subcommands and flags are constrained to benefit rapid plugin protot
 - `XCADDY_SKIP_CLEANUP=1` causes xcaddy to leave build artifacts on disk after exiting.
 - `XCADDY_WHICH_GO` sets the go command to use when for example more then 1 version of go is installed.
 - `XCADDY_GO_BUILD_FLAGS` overrides default build arguments. Supports Unix-style shell quoting, for example: XCADDY_GO_BUILD_FLAGS="-ldflags '-w s'".
+- `XCADDY_GO_MOD_CACHE_RW` sets flag `-modcacherw` before installing go modules. This sets write bit instead of read-only which is the default. See [Go 1.14 Release Notes](https://go.dev/doc/go1.14) for more information. 
 ---
 
 © 2020 Matthew Holt

builder.go

@@ -47,6 +47,7 @@ type Builder struct {
 	SkipBuild    bool          `json:"skip_build,omitempty"`
 	Debug        bool          `json:"debug,omitempty"`
 	BuildFlags   string        `json:"build_flags,omitempty"`
+	ModCacheRw   bool          `json:"mod_cache_rw,omitempty"`
 }
 
 // Build builds Caddy at the configured version with the
@@ -105,6 +106,9 @@ func (b Builder) Build(ctx context.Context, outputFile string) error {
 
 	// tidy the module to ensure go.mod and go.sum are consistent with the module prereq
 	tidyCmd := buildEnv.newCommand(utils.GetGo(), "mod", "tidy")
+	if b.ModCacheRw {
+		tidyCmd.Args = append(tidyCmd.Args, "-modcacherw")
+	}
 	if err := buildEnv.runCommand(ctx, tidyCmd, b.TimeoutGet); err != nil {
 		return err
 	}
@@ -132,6 +136,10 @@ func (b Builder) Build(ctx context.Context, outputFile string) error {
 		}
 	}
 
+	if b.ModCacheRw {
+		cmd.Args = append(cmd.Args, "-modcacherw")
+	}
+
 	if b.RaceDetector {
 		cmd.Args = append(cmd.Args, "-race")
 	}

cmd/main.go

@@ -41,6 +41,7 @@ var (
 	skipCleanup      = os.Getenv("XCADDY_SKIP_CLEANUP") == "1" || skipBuild
 	buildDebugOutput = os.Getenv("XCADDY_DEBUG") == "1"
 	buildFlags       = os.Getenv("XCADDY_GO_BUILD_FLAGS")
+	modCacheRw       = os.Getenv("XCADDY_GO_MOD_CACHE_RW") == "1"
 )
 
 func Main() {
@@ -136,6 +137,7 @@ func runBuild(ctx context.Context, args []string) error {
 		SkipCleanup:  skipCleanup,
 		Debug:        buildDebugOutput,
 		BuildFlags:   buildFlags,
+		ModCacheRw:   modCacheRw,
 	}
 	err := builder.Build(ctx, output)
 	if err != nil {

environment.go

@@ -99,6 +99,7 @@ func (b Builder) newEnvironment(ctx context.Context) (*environment, error) {
 		timeoutGoGet:    b.TimeoutGet,
 		skipCleanup:     b.SkipCleanup,
 		buildFlags:      b.BuildFlags,
+		modCacheRw:      b.ModCacheRw,
 	}
 
 	// initialize the go module
@@ -179,6 +180,7 @@ type environment struct {
 	timeoutGoGet    time.Duration
 	skipCleanup     bool
 	buildFlags      string
+	modCacheRw      bool
 }
 
 // Close cleans up the build environment, including deleting
@@ -266,11 +268,17 @@ func (env environment) execGoGet(ctx context.Context, modulePath, moduleVersion,
 	// breaks the command since it treats the empty string as a
 	// distinct argument, so we're using an if statement to avoid it.
 	var cmd *exec.Cmd
+	var args []string
+	args = append(args, "get", "-d", "-v")
+	if env.modCacheRw {
+		args = append(args, "-modcacherw")
+	}
 	if caddy != "" {
-		cmd = env.newCommand(utils.GetGo(), "get", "-d", "-v", mod, caddy)
+		args = append(args, mod, caddy)
 	} else {
-		cmd = env.newCommand(utils.GetGo(), "get", "-d", "-v", mod)
+		args = append(args, mod)
 	}
+	cmd = env.newCommand(utils.GetGo(), args...)
 
 	return env.runCommand(ctx, cmd, env.timeoutGoGet)
 }