Implement injection of a forkserver into the compiler process

Implement a forkserver inspired by AFL fuzzer. Unlike the original
design, this one relies on library preloading and seccomp to
eliminate static or dynamic recompilation of the compiler.

For single-threaded, single-process compilers this can be
several times faster, provided that the interacting
with any input files is delayed until absolutely necessary.

For example, it is the case when the compiler process first
loads the standard library for several seconds and then
processes scratch files in several milliseconds.

pmd-scm/src/main/java/net/sourceforge/pmd/scm/SourceCodeMinimizer.java

@@ -8,6 +8,7 @@
 import java.math.BigInteger;
 import java.nio.charset.Charset;
 import java.nio.file.Files;
+import java.nio.file.Path;
 import java.nio.file.StandardCopyOption;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
@@ -38,6 +39,7 @@ private static final class ExitException extends Exception { }
     private final MinimizationStrategy strategy;
     private final List<ASTCutter> cutters;
     private List<Node> currentRoots;
+    private List<SCMConfiguration.FileMapping> fileMappings;
 
     public SourceCodeMinimizer(SCMConfiguration configuration) throws IOException {
         MessageDigest md = null;
@@ -56,7 +58,8 @@ public SourceCodeMinimizer(SCMConfiguration configuration) throws IOException {
 
         Charset sourceCharset = configuration.getSourceCharset();
         cutters = new ArrayList<>();
-        for (SCMConfiguration.FileMapping mapping: configuration.getFileMappings()) {
+        fileMappings = configuration.getFileMappings();
+        for (SCMConfiguration.FileMapping mapping: fileMappings) {
             Files.copy(mapping.input, mapping.output, StandardCopyOption.REPLACE_EXISTING);
             ASTCutter cutter = new ASTCutter(parser, sourceCharset, mapping.output);
             cutters.add(cutter);
@@ -85,6 +88,15 @@ public boolean allInputsAreParseable() throws IOException {
         return true;
     }
 
+    @Override
+    public List<Path> getScratchFileNames() {
+        List<Path> result = new ArrayList<>();
+        for (SCMConfiguration.FileMapping mapping: fileMappings) {
+            result.add(mapping.output);
+        }
+        return result;
+    }
+
     @Override
     public NodeInformationProvider getNodeInformationProvider() {
         return language.getNodeInformationProvider();

pmd-scm/src/main/java/net/sourceforge/pmd/scm/invariants/AbstractExternalProcessInvariant.java

@@ -8,6 +8,7 @@
 
 import com.beust.jcommander.Parameter;
 
+import java.io.IOException;
 import java.io.PrintStream;
 
 /**
@@ -34,30 +35,40 @@ public String getName() {
         }
     }
 
-    private InvariantOperations ops;
-    private String[] commandArgs;
+    private final String compilerCommandLine;
+    protected InvariantOperations ops;
     private int spawnCount;
     private int fruitfulTests;
 
-    private static String[] createCommandLine(AbstractConfiguration configuration) {
-        if (SystemUtils.IS_OS_WINDOWS) {
-            return new String[] { "cmd.exe", "/C", configuration.compilerCommandLine };
-        } else {
-            return new String[] { "/bin/sh", "-c", configuration.compilerCommandLine };
-        }
-    }
-
     protected AbstractExternalProcessInvariant(AbstractConfiguration configuration) {
-        commandArgs = createCommandLine(configuration);
+        compilerCommandLine = configuration.compilerCommandLine;
     }
 
     @Override
-    public void initialize(InvariantOperations ops) {
+    public void initialize(InvariantOperations ops) throws IOException {
         this.ops = ops;
     }
 
     protected abstract boolean testSatisfied(ProcessBuilder pb) throws Exception;
 
+    protected String getCompilerCommandLine() {
+        return compilerCommandLine;
+    }
+
+    protected ProcessBuilder createProcessBuilder() {
+        ProcessBuilder pb = new ProcessBuilder();
+        if (SystemUtils.IS_OS_WINDOWS) {
+            pb.command("cmd.exe", "/C", compilerCommandLine);
+        } else {
+            pb.command("/bin/sh", "-c", compilerCommandLine);
+        }
+        return pb;
+    }
+
+    protected boolean testSatisfied() throws Exception {
+        return testSatisfied(createProcessBuilder());
+    }
+
     @Override
     public boolean checkIsSatisfied() throws Exception {
         // First, make a fast check that the source can be parsed at all
@@ -67,7 +78,7 @@ public boolean checkIsSatisfied() throws Exception {
 
         // then proceed to spawning subprocess
         spawnCount += 1;
-        boolean result = testSatisfied(new ProcessBuilder().command(commandArgs));
+        boolean result = testSatisfied();
         fruitfulTests += result ? 1 : 0;
 
         return result;