[gdb/testsuite] Fix gdb.threads/dlopen-libpthread.exp for upstream glibc, again

On an x86_64 laptop running ubuntu 22.04.1 with unity desktop:
...
$ echo $XDG_CURRENT_DESKTOP
Unity:Unity7:ubuntu
...
I have:
...
$ echo $LD_PRELOAD
libgtk3-nocsd.so.0
...
due to package gtk3-nocsd, a package recommended by unity-session.

Consequently, for each exec these dependencies are pulled in, including
libpthread.so.0:
...
$ lddtree /lib/x86_64-linux-gnu/libgtk3-nocsd.so.0
libgtk3-nocsd.so.0 => /lib/x86_64-linux-gnu/libgtk3-nocsd.so.0 (interpreter => none)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6
        ld-linux-x86-64.so.2 => /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
...

So, while test-case gdb.threads/dlopen-libpthread.exp appears to run ok:
...
 # of expected passes		12
 # of unsupported tests		1
...
with LD_PRELOAD="" we have instead:
...
(gdb) PASS: gdb.threads/dlopen-libpthread.exp: continue to breakpoint: notify
info sharedlibrary^M
From  To                  Syms Read   Shared Object Library^M
$hex  $hex  Yes         /lib64/ld-linux-x86-64.so.2^M
$hex  $hex  Yes         /lib/x86_64-linux-gnu/libc.so.6^M
$hex  $hex  Yes         dlopen-libpthread.so^M
(gdb) FAIL: gdb.threads/dlopen-libpthread.exp: libpthread.so found
...

The problem is that libpthread is expected as dependency of
dlopen-libpthread.so, but it's missing:
...
$ lddtree dlopen-libpthread.so
dlopen-libpthread.so => ./dlopen-libpthread.so (interpreter => none)
    libc.so.6 => $outputs/gdb.threads/dlopen-libpthread/dlopen-libpthread.so.d/libc.so.6
        ld-linux-x86-64.so.2 => /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
...
due to having glibc 2.35, which has libpthread integrated into libc.

Fix this by:
- adding a proc has_dependency
- using [has_dependency $exec libpthread.so] as hint that libpthread
  may be preloaded
- using ![has_dependency $shlib libpthread.so] to detect that
  the libpthread.so dependency is missing.

Also add a missing return after untested "no matching probes".

Tested on x86_64-linux, with and without LD_PRELOAD="".

2 files changed, 41 insertions, 6 deletions

diff --git a/gdb/testsuite/gdb.threads/dlopen-libpthread.exp b/gdb/testsuite/gdb.threads/dlopen-libpthread.exp
index 0b325ce..134265f 100644
--- a/gdb/testsuite/gdb.threads/dlopen-libpthread.exp
+++ b/gdb/testsuite/gdb.threads/dlopen-libpthread.exp
@@ -61,16 +61,32 @@ foreach probe_name $probe_names {
 
 if { !$have_probe } {
     untested "no matching probes"
+    return -1
+}
+
+# We link the exec without -lpthread, but libpthread.so may already be loaded at main
+# due to LD_PRELOAD.
+set libpthread_maybe_preloaded 0
+set binfile [standard_output_file $executable]
+if { [has_dependency $binfile libpthread\\.so] == 1 } {
+    set libpthread_maybe_preloaded 1
+}
+
+# We link the shlib with -lpthread, but since glibc 2.34 libpthread has been
+# merged with libc, so libpthread.so may not be a dependency.
+set libpthread_missing 0
+if { [has_dependency $binfile_lib libpthread\\.so] == 0 } {
+    set libpthread_missing 1
 }
 
 set test "libpthread.so not found"
 gdb_test_multiple "info sharedlibrary" $test {
     -re "/libpthread\\.so.*\r\n$gdb_prompt $" {
-	# With newer glibc, libpthread has been integrated into glibc so we
-	# can expect it to be already loaded at main.  This means we no longer
-	# excercise the scenario we're trying to trigger, but continue
-	# nevertheless.
-	unsupported $test
+	if { $libpthread_maybe_preloaded } {
+	    unsupported $test
+	} else {
+	    fail $test
+	}
     }
     -re "/libc\\.so.*\r\n$gdb_prompt $" {
 	pass $test
@@ -85,4 +101,6 @@ gdb_breakpoint "notify"
 # Cannot find new threads: generic error
 gdb_continue_to_breakpoint "notify" ".* notify-here .*"
 
-gdb_test "info sharedlibrary" {/libpthread\.so.*} "libpthread.so found"
+if { !$libpthread_missing } {
+    gdb_test "info sharedlibrary" {/libpthread\.so.*} "libpthread.so found"
+}
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index c41d469..39dfa67 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -9355,5 +9355,22 @@ proc decompress_bz2 { bz2 } {
     return $copy
 }
 
+# Return 1 if the output of "ldd FILE" contains regexp DEP, 0 if it doesn't,
+# and -1 if there was a problem running the command.
+
+proc has_dependency { file dep } {
+    set ldd [gdb_find_ldd]
+    set command "$ldd $file"
+    set result [remote_exec host $command]
+    set status [lindex $result 0]
+    set output [lindex $result 1]
+    verbose -log "status of $command is $status"
+    verbose -log "output of $command is $output"
+    if { $status != 0 || $output == "" } {
+	return -1
+    }
+    return [regexp $dep $output]
+}
+
 # Always load compatibility stuff.
 load_lib future.exp