Enable intercepting getpid()

I went with the runtime environment variable being FAKETIME_FAKEPID
since it seems less likely to collide with anything else.

Closes: #297

src/Makefile

@@ -41,6 +41,9 @@
 #     FAKE_RANDOM
 #         - Intercept getrandom()
 #
+#     FAKE_PID
+#         - Intercept getpid()
+#
 #     FORCE_MONOTONIC_FIX
 #         - If the test program hangs forever on
 #                  " pthread_cond_timedwait: CLOCK_MONOTONIC test

src/libfaketime.c

@@ -226,6 +226,9 @@ static int          (*real_futimens)        (int fd, const struct timespec times
 #ifdef FAKE_RANDOM
 static ssize_t     (*real_getrandom)        (void *buf, size_t buflen, unsigned int flags);
 #endif
+#ifdef FAKE_PID
+static pid_t       (*real_getpid)        ();
+#endif
 
 static int initialized = 0;
 
@@ -2453,6 +2456,10 @@ static void ftpl_init(void)
   real_getrandom = dlsym(RTLD_NEXT, "getrandom");
 #endif
 
+#ifdef FAKE_PID
+  real_getpid = dlsym(RTLD_NEXT, "getpid");
+#endif
+
 #ifdef FAKE_PTHREAD
 
 #ifdef __GLIBC__
@@ -3689,6 +3696,22 @@ ssize_t getrandom(void *buf, size_t buflen, unsigned int flags) {
 }
 #endif
 
+#ifdef FAKE_PID
+pid_t getpid() {
+  const char *pidstring = getenv("FAKETIME_FAKEPID");
+  if (pidstring != NULL) {
+    long int pid = strtol(pidstring, NULL, 0);
+    return (pid_t)(pid);
+  } else {
+    if (!initialized)
+      {
+        ftpl_init();
+      }
+    return real_getpid();
+  }
+}
+#endif
+
 /*
  * Editor modelines
  *

test/pidtest.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+
+FTPL="${FAKETIME_TESTLIB:-../src/libfaketime.so.1}"
+
+set -e
+run1=$(LD_PRELOAD="$FTPL" sh -c 'echo $$')
+run2=$(LD_PRELOAD="$FTPL" sh -c 'echo $$')
+
+if [ $run1 = $run2 ]; then
+   printf >&2 'got the same pid twice in a row without setting FAKETIME_FAKEPID\n'
+   exit 1
+fi
+
+output=$(FAKETIME_FAKEPID=13 LD_PRELOAD="$FTPL" sh -c 'echo $$')
+
+if [ $output != 13 ]; then
+    printf >&2 'Failed to enforce a rigid response to getpid()\n'
+    exit 2
+fi