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, 77 insertions, 0 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 0c5e360..ef3e794 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2021-01-08  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	PR gdb/27157
+	* gdb.base/empty-host-env-vars.exp: New test.
+
 2021-01-08  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.base/completion.exp: Add a new test.
diff --git a/gdb/testsuite/gdb.base/empty-host-env-vars.exp b/gdb/testsuite/gdb.base/empty-host-env-vars.exp
new file mode 100644
index 0000000..bd7212e
--- /dev/null
+++ b/gdb/testsuite/gdb.base/empty-host-env-vars.exp
@@ -0,0 +1,72 @@
+# Copyright 2021 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# GDB reads some environment variables on startup, make sure it behaves
+# correctly if these variables are defined but empty.
+
+set all_env_vars { HOME XDG_CACHE_HOME LOCALAPPDATA XDG_CONFIG_HOME }
+
+# Record the initial value of the index-cache directory.
+clean_restart
+set index_cache_directory ""
+gdb_test_multiple "show index-cache directory" "" {
+    -re -wrap "The directory of the index cache is \"(.*)\"\\." {
+	set index_cache_directory $expect_out(1,string)
+	set index_cache_directory [string_to_regexp $index_cache_directory]
+	pass $gdb_test_name
+    }
+}
+
+foreach_with_prefix env_var_name $all_env_vars {
+    # Restore the original state of the environment variable.
+    save_vars env($env_var_name) {
+	set env($env_var_name) {}
+	clean_restart
+
+	# Verify that the empty environment variable didn't affect the
+	# index-cache directory setting, that we still see the initial value.
+	# "HOME" is different, because if that  one is unset, GDB isn't even
+	# able to compute the default location.  In that case, we expect it to
+	# be empty.
+	if { $env_var_name == "HOME" } {
+	    gdb_test "show index-cache directory" \
+		"The directory of the index cache is \"\"\\."
+	} else {
+	    gdb_test "show index-cache directory" \
+		"The directory of the index cache is \"$index_cache_directory\"\\."
+	}
+    }
+}
+
+# Try the same, but with all the env vars set to an empty value at the same
+# time.
+with_test_prefix "all env vars" {
+    set save_vars_arg {}
+    foreach env_var_name $all_env_vars {
+	lappend save_vars_arg env($env_var_name)
+    }
+
+    # Restore the original state of all the environment variables.
+    save_vars $save_vars_arg {
+	foreach env_var_name $all_env_vars {
+	    set env($env_var_name) {}
+	}
+
+	clean_restart
+
+	gdb_test "show index-cache directory" \
+	    "The directory of the index cache is \"\"\\."
+    }
+}