Add more information to icingadb --version ouput

This commit adds the build information (Go version, Git commit) and system
information (distribution with version) to the output of `icingadb --version`.

When exporting the source code using `git archive`, Git will insert the version
information using the export-subst attribute (adds both a pretty version string
like `git describe` and the commit hash).

`go build` also adds some Git commit information to the resulting binary when
building from a Git working directory (only the commit hash and a flag if there
are uncommitted changes, but no pretty version string).

Note that `go build` does not include Git version information in the binary, so
when running directly from a Git working directory, commit information won't be
available in the binary, but when doing this, you should be well aware which
version you're running anyways.

System information is obtained from the `os-release` file.

.gitattributes

@@ -0,0 +1,2 @@
+# Include version information on `git archive'
+/internal/version.go export-subst

internal/command/command.go

@@ -31,7 +31,7 @@ func New() *Command {
 	}
 
 	if flags.Version {
-		fmt.Println("Icinga DB version:", internal.Version)
+		internal.Version.Print()
 		os.Exit(0)
 	}
 

internal/version.go

@@ -1,3 +1,10 @@
 package internal
 
-var Version = "v1.0.0-rc2-dev"
+import (
+	"github.com/icinga/icingadb/pkg/version"
+)
+
+// Version contains version and Git commit information.
+//
+// The placeholders are replaced on `git archive` using the `export-subst` attribute.
+var Version = version.Version("1.0.0-rc2", "$Format:%(describe)$", "$Format:%H$")

pkg/version/version.go

@@ -0,0 +1,170 @@
+package version
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"os"
+	"runtime"
+	"runtime/debug"
+	"strconv"
+	"strings"
+)
+
+type VersionInfo struct {
+	Version string
+	Commit  string
+}
+
+// Version determines version and commit information based on multiple data sources:
+//  - Version information dynamically added by `git archive` in the remaining to parameters.
+//  - A hardcoded version number passed as first parameter.
+//  - Commit information added to the binary by `go build`.
+//
+// It's supposed to be called like this in combination with setting the `export-subst` attribute for the corresponding
+// file in .gitattributes:
+//
+//     var Version = version.Version("1.0.0-rc2", "$Format:%(describe)$", "$Format:%H$")
+//
+// When exported using `git archive`, the placeholders are replaced in the file and this version information is
+// preferred. Otherwise the hardcoded version is used and augmented with commit information from the build metadata.
+func Version(version, gitDescribe, gitHash string) *VersionInfo {
+	if !strings.HasPrefix(gitDescribe, "$") && !strings.HasPrefix(gitHash, "$") {
+		return &VersionInfo{
+			Version: gitDescribe,
+			Commit:  gitHash,
+		}
+	} else {
+		commit := ""
+
+		if info, ok := debug.ReadBuildInfo(); ok {
+			modified := false
+
+			for _, setting := range info.Settings {
+				switch setting.Key {
+				case "vcs.revision":
+					commit = setting.Value
+				case "vcs.modified":
+					modified, _ = strconv.ParseBool(setting.Value)
+				}
+			}
+
+			const hashLen = 7 // Same truncation length for the commit hash as used by git describe.
+
+			if len(commit) >= hashLen {
+				version += "-g" + commit[:hashLen]
+
+				if modified {
+					version += "-dirty"
+					commit += " (modified)"
+				}
+			}
+		}
+
+		return &VersionInfo{
+			Version: version,
+			Commit:  commit,
+		}
+	}
+}
+
+// Print writes verbose version output to stdout.
+func (v *VersionInfo) Print() {
+	fmt.Println("Icinga DB version:", v.Version)
+	fmt.Println()
+
+	fmt.Println("Build information:")
+	fmt.Printf("  Go version: %s (%s, %s)\n", runtime.Version(), runtime.GOOS, runtime.GOARCH)
+	if v.Commit != "" {
+		fmt.Println("  Git commit:", v.Commit)
+	}
+
+	if r, err := readOsRelease(); err == nil {
+		fmt.Println()
+		fmt.Println("System information:")
+		fmt.Println("  Platform:", r.Name)
+		fmt.Println("  Platform version:", r.DisplayVersion())
+	}
+}
+
+// osRelease contains the information obtained from the os-release file.
+type osRelease struct {
+	Name      string
+	Version   string
+	VersionId string
+	BuildId   string
+}
+
+// DisplayVersion returns the most suitable version information for display purposes.
+func (o *osRelease) DisplayVersion() string {
+	if o.Version != "" {
+		// Most distributions set VERSION
+		return o.Version
+	} else if o.VersionId != "" {
+		// Some only set VERSION_ID (Alpine Linux for example)
+		return o.VersionId
+	} else if o.BuildId != "" {
+		// Others only set BUILD_ID (Arch Linux for example)
+		return o.BuildId
+	} else {
+		return "(unknown)"
+	}
+}
+
+// readOsRelease reads and parses the os-release file.
+func readOsRelease() (*osRelease, error) {
+	for _, path := range []string{"/etc/os-release", "/usr/lib/os-release"} {
+		f, err := os.Open(path)
+		if err != nil {
+			if os.IsNotExist(err) {
+				continue // Try next path.
+			} else {
+				return nil, err
+			}
+		}
+
+		o := &osRelease{
+			Name: "Linux", // Suggested default as per os-release(5) man page.
+		}
+
+		scanner := bufio.NewScanner(f)
+		for scanner.Scan() {
+			line := scanner.Text()
+			if strings.HasPrefix(line, "#") {
+				continue // Ignore comment.
+			}
+
+			parts := strings.SplitN(line, "=", 2)
+			if len(parts) != 2 {
+				continue // Ignore empty or possibly malformed line.
+			}
+
+			key := parts[0]
+			val := parts[1]
+
+			// Unquote strings. This isn't fully compliant with the specification which allows using some shell escape
+			// sequences. However, typically quotes are only used to allow whitespace within the value.
+			if len(val) >= 2 && (val[0] == '"' || val[0] == '\'') && val[0] == val[len(val)-1] {
+				val = val[1 : len(val)-1]
+			}
+
+			switch key {
+			case "NAME":
+				o.Name = val
+			case "VERSION":
+				o.Version = val
+			case "VERSION_ID":
+				o.VersionId = val
+			case "BUILD_ID":
+				o.BuildId = val
+			}
+		}
+		if err := scanner.Err(); err != nil {
+			return nil, err
+		}
+
+		return o, nil
+	}
+
+	return nil, errors.New("os-release file not found")
+}