pkgs/profpatsch/nman: rewrite in golang

Apart from using the go compiler, switch from `nix-shell -p`, which only worked
in certain cases, to a `nix-build` with fallback for `man` and default outputs.

Thanks to @muesli for the golang intro.

2 files changed, 140 insertions, 3 deletions

diff --git a/pkgs/profpatsch/nman/default.nix b/pkgs/profpatsch/nman/default.nix
index 10e5417f..b9b967cf 100644
--- a/pkgs/profpatsch/nman/default.nix
+++ b/pkgs/profpatsch/nman/default.nix
@@ -1,12 +1,12 @@
-{ lib, runCommandCC }:
+{ lib, runCommandNoCC, go }:
 
-runCommandCC "nman" {
+runCommandNoCC "nman" {
   meta = with lib; {
     description = "Invoke manpage in temporary nix-shell";
     license = licenses.gpl3;
   };
 } ''
-    cc -o nman ${./nman.c}
+    ${lib.getBin go}/bin/go build -o nman ${./nman.go}
     install -D nman $out/bin/nman
 ''
 
diff --git a/pkgs/profpatsch/nman/nman.go b/pkgs/profpatsch/nman/nman.go
new file mode 100644
index 00000000..e6b6247d
--- /dev/null
+++ b/pkgs/profpatsch/nman/nman.go
@@ -0,0 +1,137 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"log"
+	"bytes"
+	"os/exec"
+	"io/ioutil"
+	"syscall"
+)
+
+var usage = `nman drvAttr [section|page] [page]
+
+Open man pages in a temporary nix-shell.
+1 If one argument is given, the drvAttr & page have the same name.
+2 If two arguments are given and the second arg is
+    a <number>) like 1, but in the man section <number>
+    a <page>  ) like 1, but open the page <page>
+3 If three arguments are given, the order is <drvAttr> <sect> <page>
+`
+
+func lookupManPage(manpath string, manSection int64, manPage string) (int, error) {
+	if manSection < -1 {
+		panic("manSection must not be < -1")
+	}
+	// holy debug printf
+	// fmt.Printf("attr: %s, sect: %d, page: %s\n", drvAttr, manSection, manPage)
+
+	manBin, err := exec.LookPath("man");
+	if err != nil {
+		return 0, fmt.Errorf("man executable not in PATH")
+	}
+
+	var manArgs []string
+	if (manSection == -1) {
+		manArgs = []string{manPage}
+	} else {
+		manArgs = []string{strconv.FormatInt(manSection, 10), manPage}
+	}
+
+	man := exec.Command(manBin, manArgs...)
+
+        man.Env = append(
+		os.Environ(),
+		fmt.Sprintf("MANPATH=%s", manpath))
+	man.Stderr = os.Stderr
+	man.Stdout = os.Stdout
+	err = man.Run()
+	if exiterr, ok := err.(*exec.ExitError); ok {
+		ws := exiterr.Sys().(syscall.WaitStatus)
+		return ws.ExitStatus(), nil
+	} else {
+		return 0, err
+	}
+
+}
+
+func buildManOutput(drvAttr string) (string, error) {
+	nixExpr := fmt.Sprintf(
+		`with import <nixpkgs> {}; %s.man or %s.doc or %s.out or %s`,
+		drvAttr, drvAttr, drvAttr, drvAttr)
+
+	nixBuild := exec.Command("nix-build", "-E", nixExpr)
+	nixBuild.Stderr = os.Stderr
+	pipe, err := nixBuild.StdoutPipe()
+	if err != nil { return "", fmt.Errorf("could not access stdout of nix-build: %s", err) }
+
+	err = nixBuild.Start()
+	if err != nil { return "", fmt.Errorf("could not start nix-build: %s", err) }
+
+	out, err := ioutil.ReadAll(pipe)
+	if err != nil { return "", fmt.Errorf("could not read from nix-build: %s", err) }
+
+	lines := bytes.Split(bytes.TrimSpace(out), []byte("\n"))
+	err = nixBuild.Wait()
+	if err != nil { return "", fmt.Errorf("nix-build process died: %s", err) }
+
+	// last line ouf output looks like: /nix/store/abc…-foobar.drv!output
+	drvPath := bytes.Split(lines[len(lines)-1], []byte("!"))[0]
+	if _, err := os.Stat(string(drvPath)); err != nil {
+		return "", fmt.Errorf("%s doesn’t look like an output path: %s", drvPath, err)
+	}
+
+	return string(drvPath), nil
+}
+
+func main() {
+	var manPage string
+	var drvAttr string
+
+	args := os.Args
+
+	// man section or -1 if no man section
+        var manSection int64 = -1
+	if (len(args) >= 3) {
+		i, err := strconv.ParseUint(args[2], 10, 64)
+		if err == nil && i >= 0 {
+			manSection = int64(i)
+		}
+	}
+
+	// the first argument is always a derivation attribute
+	switch len(args) {
+	case 2:
+		// arg is package and drv attr
+		manPage = args[1]
+	case 3:
+		if (manSection == -1) {
+			// like 2, but arg 2 is package
+			manPage = args[2]
+		} else {
+			// man section given, page is arg 1
+			manPage = args[1]
+		}
+	case 4:
+		// arg 2 is manSection, arg 3 is package
+		manPage = args[3]
+	default:
+		fmt.Print(usage)
+		os.Exit(-1)
+	}
+
+	drvAttr = args[1]
+	drvPath, err := buildManOutput(drvAttr)
+	if err != nil {
+		log.Fatal(err)
+	}
+	exitStatus, err := lookupManPage(drvPath + "/share/man", manSection, manPage)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	os.Exit(exitStatus)
+
+}