fix: normalize paths on windows

Fixes #73 by normalizing paths to UNC on windows. This is _slightly_ less hacky
than registering a new schema.

The underlying issue was twofold:

1. When specifying a windows file URL as `file://c:/foo/bar`, go barfs because
`:` can't be in a domain name.
2. When specifying a windows file URL as `file:///c:/foo/bar`, we'd end up
trying to open `/c:/foo/bar` which is actually `CURRENT_DRIVE:/c:/foo/bar`.

path_other.go

@@ -0,0 +1,11 @@
+//+build !windows
+
+package log
+
+import (
+	"path/filepath"
+)
+
+func normalizePath(p string) (string, error) {
+	return filepath.Abs(p)
+}

path_windows.go

@@ -0,0 +1,35 @@
+//+build windows
+
+package log
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+)
+
+func normalizePath(p string) (string, error) {
+	if p == "" {
+		return "", fmt.Errorf("path empty")
+	}
+	p, err := filepath.Abs(p)
+	if err != nil {
+		return "", err
+	}
+	// Is this _really_ an absolute path?
+	if !strings.HasPrefix(p, "\\\\") {
+		// It's a drive: path!
+		// Return a UNC path.
+		p = "\\\\?\\" + p
+	}
+
+	// This will return file:////?/c:/foobar
+	//
+	// Why? Because:
+	//  1. Go will choke on file://c:/ because the "domain" includes a :.
+	//  2. Windows will choke on file:///c:/ because the path will be
+	//     /c:/... which is _relative_ to the current drive.
+	//
+	// This path (a) has no "domain" and (b) starts with a slash. Yay!
+	return "file://" + filepath.ToSlash(p), nil
+}

setup.go

@@ -68,7 +68,11 @@ func SetupLogging() {
 	zapCfg.OutputPaths = []string{"stderr"}
 	// check if we log to a file
 	if logfp := os.Getenv(envLoggingFile); len(logfp) > 0 {
-		zapCfg.OutputPaths = append(zapCfg.OutputPaths, logfp)
+		if path, err := normalizePath(logfp); err != nil {
+			fmt.Fprintf(os.Stderr, "failed to resolve log path '%q', logging to stderr only: %s\n", logfp, err)
+		} else {
+			zapCfg.OutputPaths = append(zapCfg.OutputPaths, path)
+		}
 	}
 
 	// set the backend(s)
@@ -83,7 +87,7 @@ func SetupLogging() {
 		var err error
 		lvl, err = LevelFromString(logenv)
 		if err != nil {
-			fmt.Println("error setting log levels", err)
+			fmt.Fprintf(os.Stderr, "error setting log levels: %s\n", err)
 		}
 	}
 	zapCfg.Level.SetLevel(zapcore.Level(lvl))