Try to use local rpm binary to query rpmdb

With this commit the rpm differ first tries to execute local rpm
to parse the image rpm database. In case of failure it fallbacks to
running the image in a container to execute the rpm shipped within
the image itself.

Signed-off-by: David Cassany <dcassany@suse.de>

Author: davidcassany

differs/rpm_diff.go

@@ -17,11 +17,14 @@ limitations under the License.
 package differs
 
 import (
+	"bufio"
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"math/rand"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"strconv"
 	"strings"
@@ -40,6 +43,11 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
+//RPM command to extract packages from the rpm database
+var rpmCmd = []string{
+	"rpm", "--nodigest", "--nosignature",
+	"-qa", "--qf", "%{NAME}\t%{VERSION}\t%{SIZE}\n",
+}
 var letters = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
 
 // daemonMutex is required to protect against other go-routines, as
@@ -87,7 +95,65 @@ func (a RPMAnalyzer) getPackages(image pkgutil.Image) (map[string]util.PackageIn
 		}
 	}
 
-	return rpmDataFromContainer(image)
+	packages, err := rpmDataFromImageFS(image)
+	if err != nil {
+		logrus.Warn("Trying to run the RPM binary of the image in a container")
+		return rpmDataFromContainer(image)
+	}
+	return packages, err
+}
+
+// rpmDataFromImageFS runs a local rpm binary, if any, to query the image
+// rpmdb and returns a map of installed packages.
+func rpmDataFromImageFS(image pkgutil.Image) (map[string]util.PackageInfo, error) {
+	packages := make(map[string]util.PackageInfo)
+	// Check there is an executable rpm tool in host
+	if err := exec.Command("rpm", "--version").Run(); err != nil {
+		logrus.Warn("No RPM binary in host")
+		return packages, err
+	}
+	dbPath, err := rpmDBPath(image.FSPath)
+	if err != nil {
+		logrus.Warnf("Couldn't find RPM database: %s", err.Error())
+		return packages, err
+	}
+	cmdArgs := append([]string{"--root", image.FSPath, "--dbpath", dbPath}, rpmCmd[1:]...)
+	out, err := exec.Command(rpmCmd[0], cmdArgs...).Output()
+	if err != nil {
+		logrus.Warnf("RPM call failed: %s", err.Error())
+		return packages, err
+	}
+	output := strings.Split(string(out), "\n")
+	return parsePackageData(output)
+}
+
+// rpmDBPath tries to get the RPM database path from the /usr/lib/rpm/macros
+// file in the image rootfs.
+func rpmDBPath(rootFSPath string) (string, error) {
+	rpmMacros, err := os.Open(filepath.Join(rootFSPath, "usr/lib/rpm/macros"))
+	if err != nil {
+		return "", err
+	}
+	defer rpmMacros.Close()
+
+	scanner := bufio.NewScanner(rpmMacros)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if strings.HasPrefix(line, "%_dbpath") {
+			fields := strings.Fields(line)
+			if len(fields) < 2 {
+				break
+			}
+			out, err := exec.Command("rpm", "-E", fields[1]).Output()
+			if err != nil {
+				return "", err
+			}
+			dbPath := strings.TrimRight(string(out), "\r\n")
+			_, err = os.Stat(filepath.Join(rootFSPath, dbPath))
+			return dbPath, err
+		}
+	}
+	return "", errors.New("Failed parsing macros file")
 }
 
 // rpmDataFromContainer runs image in a container, queries the data of
@@ -114,7 +180,7 @@ func rpmDataFromContainer(image pkgutil.Image) (map[string]util.PackageInfo, err
 	defer logrus.Infof("Removing image %s", imageName)
 
 	contConf := godocker.Config{
-		Entrypoint: []string{"rpm", "--nodigest", "--nosignature", "-qa", "--qf", "%{NAME}\t%{VERSION}\t%{SIZE}\n"},
+		Entrypoint: rpmCmd,
 		Image:      imageName,
 	}