libcpp: Fix ICE on #include after a line marker directive [PR61474]

As noted in the PR, GCC will segfault if a file name is first seen in a
linemarker directive, and then later seen in a normal #include.  This is
because the fake include process adds the file to the cache with a null PATH
member. The normal #include finds this file in the cache and then attempts
to use the null PATH.  Resolve by adding the file to the cache with a unique
starting directory, so that the fake entry will only be found by a
subsequent fake include, not by a real one.

libcpp/ChangeLog:

	PR preprocessor/61474
	* files.cc (_cpp_find_file): Set DONT_READ to TRUE for fake
	include files.
	(_cpp_fake_include): Pass a unique cpp_dir* address so
	the fake file will not be found when looked up for real.

gcc/testsuite/ChangeLog:

	PR preprocessor/61474
	* c-c++-common/cpp/pr61474-2.h: New test.
	* c-c++-common/cpp/pr61474.c: New test.
	* c-c++-common/cpp/pr61474.h: New test.

4 files changed, 21 insertions, 2 deletions

diff --git a/gcc/testsuite/c-c++-common/cpp/pr61474-2.h b/gcc/testsuite/c-c++-common/cpp/pr61474-2.h
new file mode 100644
index 0000000..6f70f09
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/pr61474-2.h
@@ -0,0 +1 @@
+#pragma once
diff --git a/gcc/testsuite/c-c++-common/cpp/pr61474.c b/gcc/testsuite/c-c++-common/cpp/pr61474.c
new file mode 100644
index 0000000..f835a40
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/pr61474.c
@@ -0,0 +1,5 @@
+/* { dg-do preprocess } */
+#include "pr61474.h"
+/* Make sure that the file can be included for real, after it was
+   fake-included by the linemarker directives in pr61474.h.  */
+#include "pr61474-2.h"
diff --git a/gcc/testsuite/c-c++-common/cpp/pr61474.h b/gcc/testsuite/c-c++-common/cpp/pr61474.h
new file mode 100644
index 0000000..d9e8c3a
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/cpp/pr61474.h
@@ -0,0 +1,6 @@
+/* Create a fake include for pr61474-2.h and exercise looking it up.  */
+/* Use #pragma once to check also that the fake-include entry in the file
+   cache does not cause a problem in libcpp/files.cc:has_unique_contents().  */
+#pragma once
+# 1 "pr61474-2.h" 1
+# 2 "pr61474-2.h" 1
diff --git a/libcpp/files.cc b/libcpp/files.cc
index 43a8894..27301d7 100644
--- a/libcpp/files.cc
+++ b/libcpp/files.cc
@@ -541,7 +541,9 @@ _cpp_find_file (cpp_reader *pfile, const char *fname, cpp_dir *start_dir,
     = (kind == _cpp_FFK_PRE_INCLUDE
        || (pfile->buffer && pfile->buffer->file->implicit_preinclude));
 
-  if (kind != _cpp_FFK_FAKE)
+  if (kind == _cpp_FFK_FAKE)
+    file->dont_read = true;
+  else
     /* Try each path in the include chain.  */
     for (;;)
       {
@@ -1490,7 +1492,12 @@ cpp_clear_file_cache (cpp_reader *pfile)
 void
 _cpp_fake_include (cpp_reader *pfile, const char *fname)
 {
-  _cpp_find_file (pfile, fname, pfile->buffer->file->dir, 0, _cpp_FFK_FAKE, 0);
+  /* It does not matter what are the contents of fake_source_dir, it will never
+     be inspected; we just use its address to uniquely signify that this file
+     was added as a fake include, so a later call to _cpp_find_file (to include
+     the file for real) won't find the fake one in the hash table.  */
+  static cpp_dir fake_source_dir;
+  _cpp_find_file (pfile, fname, &fake_source_dir, 0, _cpp_FFK_FAKE, 0);
 }
 
 /* Not everyone who wants to set system-header-ness on a buffer can