Revives the libunwind work to allow "mixed-mode" R and C profiling.

This takes a different approach than the original, only including the
native C/C++ stack *above* the last R frame -- inspired by the py-spy
project.

Partly, this concession is because merging the stacks reliably turns out
to be quite difficult, particularly due to R's lazy evaluation. But it
also reflects more closely what I think users are actually looking for
in such joint profiles.

Progress towards #8.

Makefile

@@ -1,5 +1,5 @@
 CFLAGS = -O2 -Wall -fPIC -g
-LIBS = -lelf
+LIBS = -lelf -lunwind-ptrace -lunwind-generic
 
 BIN = xrprof
 BINOBJ = src/xrprof.o

docs/xrprof.1

@@ -4,6 +4,7 @@ xrprof \- profile R programs
 .SH SYNOPSIS
 .B xrprof
 .RB [ -h ]
+.RB [ -m ]
 .RB [ -F
 .IR FREQ ]
 .RB [ -d
@@ -37,6 +38,10 @@ Set the duration for
 .B xrprof
 to take samples before exiting (provided the target program runs that
 long). The default is to last up to one hour.
+.TP
+.B \-m
+Run in \*(lqmixed mode\*(rq, where samples are drawn from both the
+R-level and native C/C++ stacks and collated together.
 .SH EXAMPLES
 Sample from an existing R program for 5 seconds at a useful frequency:
 .PP

src/xrprof.c

@@ -11,6 +11,8 @@
 #include <sys/wait.h>
 #include <time.h>    /* for timespec */
 
+#include <libunwind-ptrace.h>
+
 #include "cursor.h"
 
 #define MAX_STACK_DEPTH 100
@@ -26,18 +28,18 @@ void handle_sigint(int _sig) {
 
 void usage(const char *name) {
   // TODO: Add a long help message.
-  printf("Usage: %s [-v] [-F <freq>] [-d <duration>] -p <pid>\n", name);
+  printf("Usage: %s [-v] [-m] [-F <freq>] [-d <duration>] -p <pid>\n", name);
   return;
 }
 
 int main(int argc, char **argv) {
   pid_t pid = -1;
   int freq = DEFAULT_FREQ;
   float duration = DEFAULT_DURATION;
-  int verbose = 0;
+  int verbose = 0, mixed_mode = 0;
 
   int opt;
-  while ((opt = getopt(argc, argv, "hvF:d:p:")) != -1) {
+  while ((opt = getopt(argc, argv, "hvmF:d:p:")) != -1) {
     switch (opt) {
     case 'h':
       usage(argv[0]);
@@ -46,6 +48,9 @@ int main(int argc, char **argv) {
     case 'v':
       verbose++;
       break;
+    case 'm':
+      mixed_mode = 1;
+      break;
     case 'p':
       pid = strtol(optarg, NULL, 10);
       if ((errno == ERANGE && (pid == LONG_MAX || pid == LONG_MIN)) ||
@@ -114,6 +119,15 @@ int main(int argc, char **argv) {
     goto done;
   }
 
+  unw_addr_space_t uw_as;
+  void *uw_cxt;
+  unw_cursor_t uw_cursor;
+  if (mixed_mode) {
+    uw_as = unw_create_addr_space(&_UPT_accessors, 0);
+    unw_set_caching_policy(uw_as, UNW_CACHE_GLOBAL);
+    uw_cxt = _UPT_create(pid);
+  }
+
   /* Stop the tracee and read the R stack information. */
 
   // Allow the user to stop the tracing with Ctrl-C.
@@ -161,6 +175,76 @@ int main(int argc, char **argv) {
       goto done;
     }
 
+    if (mixed_mode && unw_init_remote(&uw_cursor, uw_as, uw_cxt) != 0) {
+      perror("fatal: Failed to initialize libunwind cursor.");
+      code++;
+      goto done;
+    } else if (mixed_mode) {
+      char sym[256];
+      unw_word_t offset, ip;
+      unw_proc_info_t info;
+
+      do {
+        sym[0] = '\0';
+        if ((ret = unw_get_proc_info(&uw_cursor, &info)) < 0) {
+          code++;
+          fprintf(stderr, "fatal: Failed to get proc info via libunwind: %d.\n",
+                  ret);
+          goto done;
+        }
+
+        if ((ret = unw_get_reg(&uw_cursor, UNW_REG_IP, &ip)) < 0) {
+          code++;
+          fprintf(stderr, "fatal: Failed to get IP register via libunwind: %d.\n",
+                  ret);
+          goto done;
+        }
+
+        if ((ret = unw_get_proc_name(&uw_cursor, sym, sizeof(sym), &offset)) < 0) {
+          if (ret == -UNW_EUNSPEC || ret == -UNW_ENOINFO) {
+            printf("\"<Native:0x%lx>\" ", ip);
+            continue;
+          } else if (ret != -UNW_ENOINFO) {
+            code++;
+            fprintf(stderr, "fatal: Failed to get proc symbol via libunwind: %d.\n",
+                    ret);
+            goto done;
+          }
+          /* Symbol is truncated but otherwise fine. */
+        }
+
+        /* We're not actually in the named procedure, but nearby.
+           TODO: The printed address is wrong; it does not account for ASLR. */
+        if (ip > info.end_ip) {
+          printf("\"<Native:0x%lx>\" ", ip);
+          continue;
+        }
+
+        /* TODO: Not sure what's going on here. */
+        if (strncmp(sym, "do_Rprof", 8) == 0) {
+          continue;
+        }
+
+        /* Look for the first eval call and jump into the R stack. */
+        if (strncmp(sym, "Rf_eval", 7) == 0) {
+          break;
+        }
+
+        /* Bail if we get to what looks like the REPL loop. */
+        if (strncmp(sym, "Rf_ReplIteration", 16) == 0) {
+          break;
+        }
+
+        printf("\"<Native:%s>\" ", sym);
+      } while ((ret = unw_step(&uw_cursor)) > 0);
+
+      if (ret < 0) {
+        code++;
+        fprintf(stderr, "fatal: Failed to step libunwind cursor: %d.\n", ret);
+        goto done;
+      }
+    }
+
     do {
       rsym[0] = '\0';
       if ((ret = xrprof_get_fun_name(cursor, rsym, sizeof(rsym))) < 0) {