Allow using local Bazel binaries. (#93)

Bazel versions (e.g. in USE_BAZEL_VERSION or .bazelversion) can now
refer to absolute paths on the filesystem. As an added convenience, a
tilde prefix is expanded to the user's home directory.

Example:

```sh
$ USE_BAZEL_VERSION="~/bin/bazel-1.0" bazelisk
```

This would tell Bazelisk to not download any binaries and instead
just directly use the Bazel binary in `$HOME/bin/bazel-1.0`.

BUILD

@@ -43,7 +43,10 @@ go_library(
     importpath = "github.com/bazelbuild/bazelisk",
     visibility = ["//visibility:private"],
     x_defs = {"BazeliskVersion": "{STABLE_VERSION}"},
-    deps = ["@com_github_hashicorp_go_version//:go_default_library"],
+    deps = [
+        "@com_github_hashicorp_go_version//:go_default_library",
+        "@com_github_mitchellh_go_homedir//:go_default_library",
+    ],
 )
 
 go_binary(

WORKSPACE

@@ -34,3 +34,10 @@ go_repository(
     sum = "h1:bPIoEKD27tNdebFGGxxYwcL4nepeY4j1QP23PFRGzg0=",
     version = "v1.1.0",
 )
+
+go_repository(
+    name = "com_github_mitchellh_go_homedir",
+    importpath = "github.com/mitchellh/go-homedir",
+    sum = "h1:lukF9ziXFxDFPkA1vsr5zpc1XuPDn/wFntq5mG+4E0Y=",
+    version = "v1.1.0",
+)

bazelisk.go

@@ -35,6 +35,7 @@ import (
 	"time"
 
 	version "github.com/hashicorp/go-version"
+	homedir "github.com/mitchellh/go-homedir"
 )
 
 const (
@@ -61,7 +62,7 @@ func findWorkspaceRoot(root string) string {
 	return findWorkspaceRoot(parentDirectory)
 }
 
-func getBazelForkAndVersion() (string, error) {
+func getBazelVersion() (string, error) {
 	// Check in this order:
 	// - env var "USE_BAZEL_VERSION" is set to a specific version.
 	// - env var "USE_NIGHTLY_BAZEL" or "USE_BAZEL_NIGHTLY" is set -> latest
@@ -686,30 +687,44 @@ func main() {
 		log.Fatalf("could not create directory %s: %v", bazeliskHome, err)
 	}
 
-	bazelForkAndVersion, err := getBazelForkAndVersion()
+	bazelVersionString, err := getBazelVersion()
 	if err != nil {
-		log.Fatalf("could not get Bazel fork and version: %v", err)
+		log.Fatalf("could not get Bazel version: %v", err)
 	}
 
-	bazelFork, bazelVersion, err := parseBazelForkAndVersion(bazelForkAndVersion)
+	bazelPath, err := homedir.Expand(bazelVersionString)
 	if err != nil {
-		log.Fatalf("could not parse Bazel fork and version: %v", err)
+		log.Fatalf("could not expand home directory in path: %v", err)
 	}
 
-	resolvedBazelVersion, isCommit, err := resolveVersionLabel(bazeliskHome, bazelFork, bazelVersion)
-	if err != nil {
-		log.Fatalf("could not resolve the version '%s' to an actual version number: %v", bazelVersion, err)
-	}
+	// If the Bazel version is an absolute path to a Bazel binary in the filesystem, we can
+	// use it directly. In that case, we don't know which exact version it is, though.
+	resolvedBazelVersion := "unknown"
+	isCommit := false
 
-	bazelDirectory := filepath.Join(bazeliskHome, "bin", bazelFork)
-	err = os.MkdirAll(bazelDirectory, 0755)
-	if err != nil {
-		log.Fatalf("could not create directory %s: %v", bazelDirectory, err)
-	}
+	// If we aren't using a local Bazel binary, we'll have to parse the version string and
+	// download the version that the user wants.
+	if !filepath.IsAbs(bazelPath) {
+		bazelFork, bazelVersion, err := parseBazelForkAndVersion(bazelVersionString)
+		if err != nil {
+			log.Fatalf("could not parse Bazel fork and version: %v", err)
+		}
 
-	bazelPath, err := downloadBazel(bazelFork, resolvedBazelVersion, isCommit, bazelDirectory)
-	if err != nil {
-		log.Fatalf("could not download Bazel: %v", err)
+		resolvedBazelVersion, isCommit, err = resolveVersionLabel(bazeliskHome, bazelFork, bazelVersion)
+		if err != nil {
+			log.Fatalf("could not resolve the version '%s' to an actual version number: %v", bazelVersion, err)
+		}
+
+		bazelDirectory := filepath.Join(bazeliskHome, "bin", bazelFork)
+		err = os.MkdirAll(bazelDirectory, 0755)
+		if err != nil {
+			log.Fatalf("could not create directory %s: %v", bazelDirectory, err)
+		}
+
+		bazelPath, err = downloadBazel(bazelFork, resolvedBazelVersion, isCommit, bazelDirectory)
+		if err != nil {
+			log.Fatalf("could not download Bazel: %v", err)
+		}
 	}
 
 	args := os.Args[1:]

go.mod

@@ -1,5 +1,8 @@
 module github.com/bazelbuild/bazelisk
 
-require github.com/hashicorp/go-version v1.1.0
+require (
+	github.com/hashicorp/go-version v1.1.0
+	github.com/mitchellh/go-homedir v1.1.0
+)
 
 go 1.13

go.sum

@@ -7,6 +7,8 @@ github.com/google/go-querystring v1.0.0 h1:Xkwi/a1rcvNg1PPYe5vI8GbeBY/jrVuDX5ASu
 github.com/google/go-querystring v1.0.0/go.mod h1:odCYkC5MyYFN7vkCjXpyrEuKhc/BUO6wN/zVPAxq5ck=
 github.com/hashicorp/go-version v1.1.0 h1:bPIoEKD27tNdebFGGxxYwcL4nepeY4j1QP23PFRGzg0=
 github.com/hashicorp/go-version v1.1.0/go.mod h1:fltr4n8CU8Ke44wwGCBoEymUuxUHl09ZGVZPK5anwXA=
+github.com/mitchellh/go-homedir v1.1.0 h1:lukF9ziXFxDFPkA1vsr5zpc1XuPDn/wFntq5mG+4E0Y=
+github.com/mitchellh/go-homedir v1.1.0/go.mod h1:SfyaCUpYCn1Vlf4IUYiD9fPX4A5wJrkLzIz1N1q0pr0=
 golang.org/x/crypto v0.0.0-20180820150726-614d502a4dac/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/net v0.0.0-20180826012351-8a410e7b638d/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
 golang.org/x/oauth2 v0.0.0-20180821212333-d2e6202438be/go.mod h1:N/0e6XlmueqKjAGxoOufVs8QHGRruUQn6yWY3a++T0U=