main: drop the dependency on llvm-ar

The ar file format is pretty simple and can be implemented by using a Go
library. Use that instead of calling out to llvm-ar.

There are a few limitations to the used package, but that doesn't seem
to matter for our use case (linking compiler-rt for use with ld.lld):

  * no index is created
  * long filenames are truncated
  * no support for archives bigger than 4GB

5 files changed, 51 insertions, 10 deletions

diff --git a/.circleci/config.yml b/.circleci/config.yml
index bc71b59d1..adb28790b 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -154,7 +154,6 @@ commands:
           name: "Create LLVM symlinks"
           command: |
             ln -s $PWD/llvm-build/bin/clang-8 /go/bin/clang-8
-            ln -s $PWD/llvm-build/bin/llvm-ar /go/bin/llvm-ar-8
             ln -s $PWD/llvm-build/bin/ld.lld  /go/bin/ld.lld-8
             ln -s $PWD/llvm-build/bin/wasm-ld /go/bin/wasm-ld-8
       - dep
@@ -220,7 +219,6 @@ commands:
           name: "Create LLVM symlinks"
           command: |
             ln -s $PWD/llvm-build/bin/clang-8 /usr/local/bin/clang-8
-            ln -s $PWD/llvm-build/bin/llvm-ar /usr/local/bin/llvm-ar
       - run:
           name: "Install Go dependencies"
           command: dep ensure --vendor-only
diff --git a/Gopkg.lock b/Gopkg.lock
index 4cf9b0f8e..e6e7dba1e 100644
--- a/Gopkg.lock
+++ b/Gopkg.lock
@@ -3,6 +3,14 @@
 
 [[projects]]
   branch = "master"
+  digest = "1:06519a2ec1d59040eaccec40206f9d0b59dc662db2a032f974d6d6b9a2bcb839"
+  name = "github.com/blakesmith/ar"
+  packages = ["."]
+  pruneopts = "UT"
+  revision = "8bd4349a67f2533b078dbc524689d15dba0f4659"
+
+[[projects]]
+  branch = "master"
   digest = "1:00b45e06c7843541372fc17d982242bd6adfc2fc382b6f2e9ef9ce53d87a50b9"
   name = "github.com/marcinbor85/gohex"
   packages = ["."]
@@ -33,6 +41,7 @@
   analyzer-name = "dep"
   analyzer-version = 1
   input-imports = [
+    "github.com/blakesmith/ar",
     "github.com/marcinbor85/gohex",
     "golang.org/x/tools/go/ast/astutil",
     "golang.org/x/tools/go/ssa",
diff --git a/builtins.go b/builtins.go
index 2ec155884..a34001a77 100644
--- a/builtins.go
+++ b/builtins.go
@@ -1,11 +1,16 @@
 package main
 
 import (
+	"errors"
+	"io"
 	"io/ioutil"
 	"os"
 	"os/exec"
 	"path/filepath"
 	"strings"
+	"time"
+
+	"github.com/blakesmith/ar"
 )
 
 // These are the GENERIC_SOURCES according to CMakeList.txt.
@@ -246,14 +251,45 @@ func compileBuiltins(target string, callback func(path string) error) error {
 	}
 
 	// Put all builtins in an archive to link as a static library.
+	// Note: this does not create a symbol index, but ld.lld doesn't seem to
+	// care.
 	arpath := filepath.Join(dir, "librt.a")
-	cmd := exec.Command(commands["ar"], append([]string{"cr", arpath}, objs...)...)
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
-	cmd.Dir = dir
-	err = cmd.Run()
+	arfile, err := os.Create(arpath)
+	if err != nil {
+		return err
+	}
+	defer arfile.Close()
+	arwriter := ar.NewWriter(arfile)
+	err = arwriter.WriteGlobalHeader()
 	if err != nil {
-		return &commandError{"failed to make static library", arpath, err}
+		return &os.PathError{"write ar header", arpath, err}
+	}
+	for _, objpath := range objs {
+		name := filepath.Base(objpath)
+		objfile, err := os.Open(objpath)
+		if err != nil {
+			return err
+		}
+		defer objfile.Close()
+		st, err := objfile.Stat()
+		if err != nil {
+			return err
+		}
+		arwriter.WriteHeader(&ar.Header{
+			Name:    name,
+			ModTime: time.Unix(0, 0),
+			Uid:     0,
+			Gid:     0,
+			Mode:    0644,
+			Size:    st.Size(),
+		})
+		n, err := io.Copy(arwriter, objfile)
+		if err != nil {
+			return err
+		}
+		if n != st.Size() {
+			return errors.New("file modified during ar creation: " + arpath)
+		}
 	}
 
 	// Give the caller the resulting file. The callback must copy the file,
diff --git a/commands.go b/commands.go
index 8fb9d13da..a975b5b2c 100644
--- a/commands.go
+++ b/commands.go
@@ -4,7 +4,6 @@ package main
 
 // commands used by the compilation process might have different file names on Linux than those used on macOS.
 var commands = map[string]string{
-	"ar":      "llvm-ar-8",
 	"clang":   "clang-8",
 	"ld.lld":  "ld.lld-8",
 	"wasm-ld": "wasm-ld-8",
diff --git a/commands_macos.go b/commands_macos.go
index e457ba886..9ec428ec6 100644
--- a/commands_macos.go
+++ b/commands_macos.go
@@ -4,7 +4,6 @@ package main
 
 // commands used by the compilation process might have different file names on macOS than those used on Linux.
 var commands = map[string]string{
-	"ar":      "llvm-ar",
 	"clang":   "clang-8",
 	"ld.lld":  "ld.lld",
 	"wasm-ld": "wasm-ld",