[gdb/testsuite] Fix "text file busy" errors with cc-with-tweaks.exp

When using target board cc-with-gdb-index.exp and running tests in parallel,
we run into:
...
gdb compile failed, gdb/contrib/gdb-add-index.sh: line 86: \
  build/gdb/testsuite/gdb.sh: Text file busy
...

The problem is that because of the parallel test run, gdb.sh is created for
every single test-case, and eventually gdb.sh is overwritten while being
executed.

Fix this by creating gdb.sh only once.

Tested on x86_64-linux with target board cc-with-gdb-index.exp, using both a
serial and parallel -j 5 test run.

gdb/testsuite/ChangeLog:

2020-03-06  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (tentative_rename, cached_file): New proc.
	* boards/cc-with-tweaks.exp: Use cached_file to create gdb.sh.

3 files changed, 49 insertions, 4 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index f1c9b2c..912cdf3 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2020-03-06  Tom de Vries  <tdevries@suse.de>
 
+	* lib/gdb.exp (tentative_rename, cached_file): New proc.
+	* boards/cc-with-tweaks.exp: Use cached_file to create gdb.sh.
+
+2020-03-06  Tom de Vries  <tdevries@suse.de>
+
 	* README: Fix "the the".
 	* gdb.base/dprintf.exp: Same.
 
diff --git a/gdb/testsuite/boards/cc-with-tweaks.exp b/gdb/testsuite/boards/cc-with-tweaks.exp
index a60507a..8701ccd 100644
--- a/gdb/testsuite/boards/cc-with-tweaks.exp
+++ b/gdb/testsuite/boards/cc-with-tweaks.exp
@@ -69,7 +69,5 @@ if ![info exists F77_FOR_TARGET] {
 }
 set F77_FOR_TARGET "$contrib_dir/cc-with-tweaks.sh $CC_WITH_TWEAKS_FLAGS $F77_FOR_TARGET"
 
-set pwd [exec pwd -P]
-exec echo $GDB $INTERNAL_GDBFLAGS $GDBFLAGS \"\$@\" > $pwd/gdb.sh
-exec chmod +x gdb.sh
-set env(GDB) $pwd/gdb.sh
+set env(GDB) \
+    [cached_file gdb.sh "$GDB $INTERNAL_GDBFLAGS $GDBFLAGS \"\$@\"" 1]
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index f8f404f..55e5858 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -4929,6 +4929,48 @@ proc standard_temp_file {basename} {
     return [file join $dir $basename]
 }
 
+# Rename file A to file B, if B does not already exists.  Otherwise, leave B
+# as is and delete A.  Return 1 if rename happened.
+
+proc tentative_rename { a b } {
+    global errorInfo errorCode
+    set code [catch {file rename -- $a $b} result]
+    if { $code == 1 && [lindex $errorCode 0] == "POSIX" \
+	     && [lindex $errorCode 1] == "EEXIST" } {
+	file delete $a
+	return 0
+    }
+    if {$code == 1} {
+	return -code error -errorinfo $errorInfo -errorcode $errorCode $result
+    } elseif {$code > 1} {
+	return -code $code $result
+    }
+    return 1
+}
+
+# Create a file with name FILENAME and contents TXT in the cache directory.
+# If EXECUTABLE, mark the new file for execution.
+
+proc cached_file { filename txt {executable 0}} {
+    set filename [make_gdb_parallel_path cache $filename]
+
+    if { [file exists $filename] } {
+	return $filename
+    }
+
+    set tmp_filename $filename.[pid]
+    set fd [open $tmp_filename w]
+    puts $fd $txt
+    close $fd
+
+    if { $executable } {
+	exec chmod +x $tmp_filename
+    }
+    tentative_rename $tmp_filename $filename
+
+    return $filename
+}
+
 # Set 'testfile', 'srcfile', and 'binfile'.
 #
 # ARGS is a list of source file specifications.