Reimplement mountinfo parsing without using regex

Author: pejovica

src/java.base/linux/classes/jdk/internal/platform/CgroupSubsystemFactory.java

@@ -38,8 +38,6 @@
 import java.util.Optional;
 import java.util.Objects;
 import java.util.function.Consumer;
-import java.util.regex.Matcher;
-import java.util.regex.Pattern;
 import java.util.stream.Stream;
 
 import jdk.internal.platform.cgroupv1.CgroupV1Subsystem;
@@ -54,31 +52,6 @@ public class CgroupSubsystemFactory {
     private static final String MEMORY_CTRL = "memory";
     private static final String PIDS_CTRL = "pids";
 
-    /*
-     * From https://www.kernel.org/doc/Documentation/filesystems/proc.txt
-     *
-     *  36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
-     *  (1)(2)(3)   (4)   (5)      (6)      (7)   (8) (9)   (10)         (11)
-     *
-     *  (1) mount ID:  unique identifier of the mount (may be reused after umount)
-     *  (2) parent ID:  ID of parent (or of self for the top of the mount tree)
-     *  (3) major:minor:  value of st_dev for files on filesystem
-     *  (4) root:  root of the mount within the filesystem
-     *  (5) mount point:  mount point relative to the process's root
-     *  (6) mount options:  per mount options
-     *  (7) optional fields:  zero or more fields of the form "tag[:value]"
-     *  (8) separator:  marks the end of the optional fields
-     *  (9) filesystem type:  name of filesystem of the form "type[.subtype]"
-     *  (10) mount source:  filesystem specific information or "none"
-     *  (11) super options:  per super block options
-     */
-    private static final Pattern MOUNTINFO_PATTERN = Pattern.compile(
-        "^[^\\s]+\\s+[^\\s]+\\s+[^\\s]+\\s+" + // (1), (2), (3)
-        "([^\\s]+)\\s+([^\\s]+)\\s+" +         // (4), (5)     - group 1, 2: root, mount point
-        "[^-]+-\\s+" +                         // (6), (7), (8)
-        "([^\\s]+)\\s+" +                      // (9)          - group 3: filesystem type
-        ".*$");                                // (10), (11)
-
     static CgroupMetrics create() {
         Optional<CgroupTypeResult> optResult = null;
         try {
@@ -285,8 +258,8 @@ private static void setCgroupV1Path(Map<String, CgroupInfo> infos,
     /**
      * Amends cgroup infos with mount path and mount root. The passed in
      * 'mntInfoLine' represents a single line in, for example,
-     * /proc/self/mountinfo. Each line is matched with MOUNTINFO_PATTERN
-     * (see above), so as to extract the relevant tokens from the line.
+     * /proc/self/mountinfo. Each line is parsed with {@link MountInfo#parse},
+     * so as to extract the relevant tokens from the line.
      *
      * Host example cgroups v1:
      *
@@ -307,13 +280,13 @@ private static void setCgroupV1Path(Map<String, CgroupInfo> infos,
     private static boolean amendCgroupInfos(String mntInfoLine,
                                             Map<String, CgroupInfo> infos,
                                             boolean isCgroupsV2) {
-        Matcher lineMatcher = MOUNTINFO_PATTERN.matcher(mntInfoLine.trim());
+        MountInfo mountInfo = MountInfo.parse(mntInfoLine);
         boolean cgroupv1ControllerFound = false;
         boolean cgroupv2ControllerFound = false;
-        if (lineMatcher.matches()) {
-            String mountRoot = lineMatcher.group(1);
-            String mountPath = lineMatcher.group(2);
-            String fsType = lineMatcher.group(3);
+        if (mountInfo != null) {
+            String mountRoot = mountInfo.mountRoot;
+            String mountPath = mountInfo.mountPath;
+            String fsType = mountInfo.fsType;
             if (fsType.equals("cgroup")) {
                 Path p = Paths.get(mountPath);
                 String[] controllerNames = p.getFileName().toString().split(",");
@@ -349,6 +322,59 @@ private static boolean amendCgroupInfos(String mntInfoLine,
         return cgroupv1ControllerFound || cgroupv2ControllerFound;
     }
 
+    private record MountInfo(String mountRoot, String mountPath, String fsType) {
+        /*
+         * From https://www.kernel.org/doc/Documentation/filesystems/proc.txt
+         *
+         *  36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
+         *  (1)(2)(3)   (4)   (5)      (6)      (7)   (8) (9)   (10)         (11)
+         *
+         *  (1) mount ID:  unique identifier of the mount (may be reused after umount)
+         *  (2) parent ID:  ID of parent (or of self for the top of the mount tree)
+         *  (3) major:minor:  value of st_dev for files on filesystem
+         *  (4) root:  root of the mount within the filesystem
+         *  (5) mount point:  mount point relative to the process's root
+         *  (6) mount options:  per mount options
+         *  (7) optional fields:  zero or more fields of the form "tag[:value]"
+         *  (8) separator:  marks the end of the optional fields
+         *  (9) filesystem type:  name of filesystem of the form "type[.subtype]"
+         *  (10) mount source:  filesystem specific information or "none"
+         *  (11) super options:  per super block options
+         */
+        static MountInfo parse(String line) {
+            String mountRoot = null;
+            String mountPath = null;
+
+            int separatorOrdinal = -1;
+            // loop over space-separated tokens
+            for (int tOrdinal = 1, tStart = 0, tEnd = line.indexOf(' '); tEnd != -1; tOrdinal++, tStart = tEnd + 1, tEnd = line.indexOf(' ', tStart)) {
+                if (tStart == tEnd) {
+                    break; // unexpected empty token
+                }
+                switch (tOrdinal) {
+                    case 1, 2, 3, 6 -> {} // skip token
+                    case 4 -> mountRoot = line.substring(tStart, tEnd); // root token
+                    case 5 -> mountPath = line.substring(tStart, tEnd); // mount point token
+                    default -> {
+                        assert tOrdinal >= 7;
+                        if (separatorOrdinal == -1) {
+                            // check if we found a separator token
+                            if (tEnd - tStart == 1 && line.charAt(tStart) == '-') {
+                                separatorOrdinal = tOrdinal;
+                            }
+                            continue; // skip token
+                        }
+                        if (tOrdinal == separatorOrdinal + 1) { // filesystem type token
+                            String fsType = line.substring(tStart, tEnd);
+                            return new MountInfo(mountRoot, mountPath, fsType);
+                        }
+                    }
+                }
+            }
+            return null; // parsing failed
+        }
+    }
+
     private static void setMountPoints(CgroupInfo info, String mountPath, String mountRoot) {
         if (info.getMountPoint() != null) {
             // On some systems duplicate controllers get mounted in addition to