gdb/testsuite: Installed-GDB testing & data-directory

In testsuite/README, we suggest that you can run the testsuite against
some other GDB binary by using:

    make check RUNTESTFLAGS=GDB=/usr/bin/gdb

However, that example isn't fully correct, because with that command
line, the testsuite will still pass

  -data-directory=[pwd]/../data-directory

to /usr/bin/gdb, like e.g.:

  ...
  builtin_spawn /usr/bin/gdb -nw -nx -data-directory /home/pedro/gdb/build/gdb/testsuite/../data-directory -iex set height 0 -iex set width 0
  ...

while if you're testing an installed GDB (the system GDB being the
most usual scenario), then you should normally let it use its own
configured directory, not the just-built GDB's data directory.

This commit improves the status quo with the following two changes:

 - if the user specifies GDB on the command line, then by default,
   don't start GDB with the -data-directory command line option.
   I.e., let the tested GDB use its own configured data directory.

 - let the user override the data directory, via a new
   GDB_DATA_DIRECTORY global.  This replaces the existing
   BUILD_DATA_DIRECTORY variable in testsuite/lib/gdb.exp, which
   wasn't overridable, and was a bit misnamed for the new purpose.

So after this, the following commands I believe behave intuitively:

 # Test the non-installed GDB in some build dir:

    make check \
      RUNTESTFLAGS="GDB=/path/to/other/build/gdb \
                    GDB_DATA_DIRECTORY=/path/to/other/build/gdb/data-directory"

 # Test the GDB installed in some prefix:

    make check \
      RUNTESTFLAGS="GDB=/opt/gdb/bin/gdb"

 # Test the built GDB with some alternative data directory, e.g., the
   system GDB's data directory:

    make check \
      RUNTESTFLAGS="GDB_DATA_DIRECTORY=/usr/share/gdb"

Change-Id: Icdc21c85219155d9564a9900961997e6624b78fb

gdb/testsuite/README

@@ -164,6 +164,19 @@ instance
 
 checks both the installed GDB and GDBserver.
 
+GDB_DATA_DIRECTORY
+
+If you set GDB, then by default the testsuite assumes you are
+exercising an installed GDB, and thus the testsuite lets GDB use its
+configured data directory.  Otherwise, if you don't set GDB, then by
+default the tested GDB uses the data directory found under the GDB
+build directory.  You can override this by setting GDB_DATA_DIRECTORY.
+For instance:
+
+    make check \
+      RUNTESTFLAGS="GDB=/path/to/other/build/gdb \
+                    GDB_DATA_DIRECTORY=/path/to/other/build/gdb/data-directory"
+
 INTERNAL_GDBFLAGS
 
 Command line options passed to all GDB invocations.

gdb/testsuite/lib/gdb.exp

@@ -92,8 +92,13 @@ load_lib gdb-utils.exp
 load_lib memory.exp
 load_lib check-test-names.exp
 
+# The path to the GDB binary to test.
 global GDB
 
+# The data directory to use for testing.  If this is the empty string,
+# then we let GDB use its own configured data directory.
+global GDB_DATA_DIRECTORY
+
 # The spawn ID used for I/O interaction with the inferior.  For native
 # targets, or remote targets that can do I/O through GDB
 # (semi-hosting) this will be the same as the host/GDB's spawn ID.
@@ -111,9 +116,24 @@ if ![info exists GDB] {
     } else {
 	set GDB [transform gdb]
     }
+} else {
+    # If the user specifies GDB on the command line, and doesn't
+    # specify GDB_DATA_DIRECTORY, then assume we're testing an
+    # installed GDB, and let it use its own configured data directory.
+    if ![info exists GDB_DATA_DIRECTORY] {
+	set GDB_DATA_DIRECTORY ""
+    }
 }
 verbose "using GDB = $GDB" 2
 
+# The data directory the testing GDB will use.  By default, assume
+# we're testing a non-installed GDB in the build directory.  Users may
+# also explictly override the -data-directory from the command line.
+if ![info exists GDB_DATA_DIRECTORY] {
+    set GDB_DATA_DIRECTORY "[pwd]/../data-directory"
+}
+verbose "using GDB_DATA_DIRECTORY = $GDB_DATA_DIRECTORY" 2
+
 # GDBFLAGS is available for the user to set on the command line.
 # E.g. make check RUNTESTFLAGS=GDBFLAGS=mumble
 # Testcases may use it to add additional flags, but they must:
@@ -125,23 +145,35 @@ if ![info exists GDBFLAGS] {
 }
 verbose "using GDBFLAGS = $GDBFLAGS" 2
 
-# Make the build data directory available to tests.
-set BUILD_DATA_DIRECTORY "[pwd]/../data-directory"
+# Append the -data-directory option to pass to GDB to CMDLINE and
+# return the resulting string.  If GDB_DATA_DIRECTORY is empty,
+# nothing is appended.
+proc append_gdb_data_directory_option {cmdline} {
+    global GDB_DATA_DIRECTORY
+
+    if { $GDB_DATA_DIRECTORY != "" } {
+	return "$cmdline -data-directory $GDB_DATA_DIRECTORY"
+    } else {
+	return $cmdline
+    }
+}
 
 # INTERNAL_GDBFLAGS contains flags that the testsuite requires.
 # `-nw' disables any of the windowed interfaces.
 # `-nx' disables ~/.gdbinit, so that it doesn't interfere with the tests.
-# `-data-directory' points to the data directory in the build directory.
 # `-iex "set {height,width} 0"' disables pagination.
+# `-data-directory' points to the data directory, usually in the build
+# directory.
 global INTERNAL_GDBFLAGS
 if ![info exists INTERNAL_GDBFLAGS] {
     set INTERNAL_GDBFLAGS \
 	[join [list \
 		   "-nw" \
 		   "-nx" \
-		   "-data-directory $BUILD_DATA_DIRECTORY" \
 		   {-iex "set height 0"} \
 		   {-iex "set width 0"}]]
+
+    set INTERNAL_GDBFLAGS [append_gdb_data_directory_option $INTERNAL_GDBFLAGS]
 }
 
 # The variable gdb_prompt is a regexp which matches the gdb prompt.
@@ -8021,9 +8053,9 @@ proc verify_psymtab_expanded { filename readin } {
 # string (the default) means .gdb_index; "-dwarf-5" means .debug_names.
 
 proc add_gdb_index { program {style ""} } {
-    global srcdir GDB env BUILD_DATA_DIRECTORY
+    global srcdir GDB env
     set contrib_dir "$srcdir/../contrib"
-    set env(GDB) "$GDB --data-directory=$BUILD_DATA_DIRECTORY"
+    set env(GDB) [append_gdb_data_directory_option $GDB]
     set result [catch "exec $contrib_dir/gdb-add-index.sh $style $program" output]
     if { $result != 0 } {
 	verbose -log "result is $result"