gdb: check for empty strings in get_standard_cache_dir/get_standard_config_dir

As reported in PR 27157, if some environment variables read at startup
by GDB are defined but empty, we hit the assert in gdb_abspath:

    $ XDG_CACHE_HOME= ./gdb -nx --data-directory=data-directory -q
    AddressSanitizer:DEADLYSIGNAL
    =================================================================
    ==2007040==ERROR: AddressSanitizer: SEGV on unknown address 0x0000000001b0 (pc 0x5639d4aa4127 bp 0x7ffdac232c00 sp 0x7ffdac232bf0 T0)
    ==2007040==The signal is caused by a READ memory access.
    ==2007040==Hint: address points to the zero page.
        #0 0x5639d4aa4126 in target_stack::top() const /home/smarchi/src/binutils-gdb/gdb/target.h:1334
        #1 0x5639d4aa41f1 in inferior::top_target() /home/smarchi/src/binutils-gdb/gdb/inferior.h:369
        #2 0x5639d4a70b1f in current_top_target() /home/smarchi/src/binutils-gdb/gdb/target.c:120
        #3 0x5639d4b00591 in gdb_readline_wrapper_cleanup::gdb_readline_wrapper_cleanup() /home/smarchi/src/binutils-gdb/gdb/top.c:1046
        #4 0x5639d4afab31 in gdb_readline_wrapper(char const*) /home/smarchi/src/binutils-gdb/gdb/top.c:1104
        #5 0x5639d4ccce2c in defaulted_query /home/smarchi/src/binutils-gdb/gdb/utils.c:893
        #6 0x5639d4ccd6af in query(char const*, ...) /home/smarchi/src/binutils-gdb/gdb/utils.c:985
        #7 0x5639d4ccaec1 in internal_vproblem /home/smarchi/src/binutils-gdb/gdb/utils.c:373
        #8 0x5639d4ccb3d1 in internal_verror(char const*, int, char const*, __va_list_tag*) /home/smarchi/src/binutils-gdb/gdb/utils.c:439
        #9 0x5639d5151a92 in internal_error(char const*, int, char const*, ...) /home/smarchi/src/binutils-gdb/gdbsupport/errors.cc:55
        #10 0x5639d5162ab4 in gdb_abspath(char const*) /home/smarchi/src/binutils-gdb/gdbsupport/pathstuff.cc:132
        #11 0x5639d5162fac in get_standard_cache_dir[abi:cxx11]() /home/smarchi/src/binutils-gdb/gdbsupport/pathstuff.cc:228
        #12 0x5639d3e76a81 in _initialize_index_cache() /home/smarchi/src/binutils-gdb/gdb/dwarf2/index-cache.c:325
        #13 0x5639d4dbbe92 in initialize_all_files() /home/smarchi/build/binutils-gdb/gdb/init.c:321
        #14 0x5639d4b00259 in gdb_init(char*) /home/smarchi/src/binutils-gdb/gdb/top.c:2344
        #15 0x5639d4440715 in captured_main_1 /home/smarchi/src/binutils-gdb/gdb/main.c:950
        #16 0x5639d444252e in captured_main /home/smarchi/src/binutils-gdb/gdb/main.c:1229
        #17 0x5639d44425cf in gdb_main(captured_main_args*) /home/smarchi/src/binutils-gdb/gdb/main.c:1254
        #18 0x5639d3923371 in main /home/smarchi/src/binutils-gdb/gdb/gdb.c:32
        #19 0x7fa002d3f0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
        #20 0x5639d392314d in _start (/home/smarchi/build/binutils-gdb/gdb/gdb+0x4d414d)

gdb_abspath doesn't handle empty strings, so handle this case in the
callers.  If a variable is defined but empty, I think it's reasonable in
this case to just ignore it, as if it was not defined.

Note that this sometimes also lead to a segfault, because the failed
assertion happens very early during startup, before things are fully
initialized.

gdbsupport/ChangeLog:

	PR gdb/27157
	* pathstuff.cc (get_standard_cache_dir, get_standard_config_dir,
	find_gdb_home_config_file): Add empty string check.

gdb/testsuite/ChangeLog:

	PR gdb/27157
	* gdb.base/empty-host-env-vars.exp: New test.

Change-Id: I8654d8e97e74e1dff6d308c111ae4b1bbf07bef9

2 files changed, 13 insertions, 7 deletions

diff --git a/gdbsupport/ChangeLog b/gdbsupport/ChangeLog
index 4cac86f..d973a6d 100644
--- a/gdbsupport/ChangeLog
+++ b/gdbsupport/ChangeLog
@@ -1,3 +1,9 @@
+2021-01-08  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	PR gdb/27157
+	* pathstuff.cc (get_standard_cache_dir, get_standard_config_dir,
+	find_gdb_home_config_file): Add empty string check.
+
 2021-01-06  Mike Frysinger  <vapier@gentoo.org>
 
 	* common-utils.h (align_up): Fix typo in header comment.
diff --git a/gdbsupport/pathstuff.cc b/gdbsupport/pathstuff.cc
index 51ab8c6..ad13900 100644
--- a/gdbsupport/pathstuff.cc
+++ b/gdbsupport/pathstuff.cc
@@ -222,7 +222,7 @@ get_standard_cache_dir ()
 
 #ifndef __APPLE__
   const char *xdg_cache_home = getenv ("XDG_CACHE_HOME");
-  if (xdg_cache_home != NULL)
+  if (xdg_cache_home != NULL && xdg_cache_home[0] != '\0')
     {
       /* Make sure the path is absolute and tilde-expanded.  */
       gdb::unique_xmalloc_ptr<char> abs (gdb_abspath (xdg_cache_home));
@@ -231,7 +231,7 @@ get_standard_cache_dir ()
 #endif
 
   const char *home = getenv ("HOME");
-  if (home != NULL)
+  if (home != NULL && home[0] != '\0')
     {
       /* Make sure the path is absolute and tilde-expanded.  */
       gdb::unique_xmalloc_ptr<char> abs (gdb_abspath (home));
@@ -240,14 +240,14 @@ get_standard_cache_dir ()
 
 #ifdef WIN32
   const char *win_home = getenv ("LOCALAPPDATA");
-  if (win_home != NULL)
+  if (win_home != NULL && win_home[0] != '\0')
     {
       /* Make sure the path is absolute and tilde-expanded.  */
       gdb::unique_xmalloc_ptr<char> abs (gdb_abspath (win_home));
       return string_printf ("%s/gdb", abs.get ());
     }
 #endif
-    
+
   return {};
 }
 
@@ -289,7 +289,7 @@ get_standard_config_dir ()
 
 #ifndef __APPLE__
   const char *xdg_config_home = getenv ("XDG_CONFIG_HOME");
-  if (xdg_config_home != NULL)
+  if (xdg_config_home != NULL && xdg_config_home[0] != '\0')
     {
       /* Make sure the path is absolute and tilde-expanded.  */
       gdb::unique_xmalloc_ptr<char> abs (gdb_abspath (xdg_config_home));
@@ -298,7 +298,7 @@ get_standard_config_dir ()
 #endif
 
   const char *home = getenv ("HOME");
-  if (home != NULL)
+  if (home != NULL && home[0] != '\0')
     {
       /* Make sure the path is absolute and tilde-expanded.  */
       gdb::unique_xmalloc_ptr<char> abs (gdb_abspath (home));
@@ -340,7 +340,7 @@ find_gdb_home_config_file (const char *name, struct stat *buf)
     }
 
   const char *homedir = getenv ("HOME");
-  if (homedir != nullptr)
+  if (homedir != nullptr && homedir[0] != '\0')
     {
       /* Make sure the path is absolute and tilde-expanded.  */
       gdb::unique_xmalloc_ptr<char> abs (gdb_abspath (homedir));