diff --git a/t/README b/t/README
index 28711cc508..11ce7675e3 100644
--- a/t/README
+++ b/t/README
@@ -186,6 +186,22 @@ appropriately before running "make".
 	this feature by setting the GIT_TEST_CHAIN_LINT environment
 	variable to "1" or "0", respectively.
 
+--stress::
+--stress=<N>::
+	Run the test script repeatedly in multiple parallel jobs until
+	one of them fails.  Useful for reproducing rare failures in
+	flaky tests.  The number of parallel jobs is, in order of
+	precedence: <N>, or the value of the GIT_TEST_STRESS_LOAD
+	environment variable, or twice the number of available
+	processors (as shown by the 'getconf' utility),	or 8.
+	Implies `--verbose -x --immediate` to get the most information
+	about the failure.  Note that the verbose output of each test
+	job is saved to 't/test-results/$TEST_NAME.stress-<nr>.out',
+	and only the output of the failed test job is shown on the
+	terminal.  The names of the trash directories get a
+	'.stress-<nr>' suffix, and the trash directory of the failed
+	test job is renamed to end with a '.stress-failed' suffix.
+
 You can also set the GIT_TEST_INSTALLED environment variable to
 the bindir of an existing git installation to test that installation.
 You still need to have built this git sandbox, from which various
@@ -425,7 +441,8 @@ This test harness library does the following things:
  - Creates an empty test directory with an empty .git/objects database
    and chdir(2) into it.  This directory is 't/trash
    directory.$test_name_without_dotsh', with t/ subject to change by
-   the --root option documented above.
+   the --root option documented above, and a '.stress-<N>' suffix
+   appended by the --stress option.
 
  - Defines standard test helper functions for your scripts to
    use.  These functions are designed to make all scripts behave
diff --git a/t/lib-git-daemon.sh b/t/lib-git-daemon.sh
index fd41229a8f..79db3b7ae5 100644
--- a/t/lib-git-daemon.sh
+++ b/t/lib-git-daemon.sh
@@ -28,7 +28,7 @@ then
 	test_skip_or_die $GIT_TEST_GIT_DAEMON "file system does not support FIFOs"
 fi
 
-LIB_GIT_DAEMON_PORT=${LIB_GIT_DAEMON_PORT-${this_test#t}}
+test_set_port LIB_GIT_DAEMON_PORT
 
 GIT_DAEMON_PID=
 GIT_DAEMON_DOCUMENT_ROOT_PATH="$PWD"/repo
diff --git a/t/lib-git-p4.sh b/t/lib-git-p4.sh
index c27599474c..b3be3ba011 100644
--- a/t/lib-git-p4.sh
+++ b/t/lib-git-p4.sh
@@ -53,14 +53,7 @@ time_in_seconds () {
 	(cd / && "$PYTHON_PATH" -c 'import time; print(int(time.time()))')
 }
 
-# Try to pick a unique port: guess a large number, then hope
-# no more than one of each test is running.
-#
-# This does not handle the case where somebody else is running the
-# same tests and has chosen the same ports.
-testid=${this_test#t}
-git_p4_test_start=9800
-P4DPORT=$((10669 + ($testid - $git_p4_test_start)))
+test_set_port P4DPORT
 
 P4PORT=localhost:$P4DPORT
 P4CLIENT=client
diff --git a/t/lib-git-svn.sh b/t/lib-git-svn.sh
index a8130f9119..f3b478c307 100644
--- a/t/lib-git-svn.sh
+++ b/t/lib-git-svn.sh
@@ -13,6 +13,7 @@ fi
 GIT_DIR=$PWD/.git
 GIT_SVN_DIR=$GIT_DIR/svn/refs/remotes/git-svn
 SVN_TREE=$GIT_SVN_DIR/svn-tree
+test_set_port SVNSERVE_PORT
 
 svn >/dev/null 2>&1
 if test $? -ne 1
@@ -119,7 +120,6 @@ require_svnserve () {
 }
 
 start_svnserve () {
-	SVNSERVE_PORT=${SVNSERVE_PORT-${this_test#t}}
 	svnserve --listen-port $SVNSERVE_PORT \
 		 --root "$rawsvnrepo" \
 		 --listen-once \
diff --git a/t/lib-httpd.sh b/t/lib-httpd.sh
index a8729f8232..e465116ef9 100644
--- a/t/lib-httpd.sh
+++ b/t/lib-httpd.sh
@@ -82,7 +82,7 @@ case $(uname) in
 esac
 
 LIB_HTTPD_PATH=${LIB_HTTPD_PATH-"$DEFAULT_HTTPD_PATH"}
-LIB_HTTPD_PORT=${LIB_HTTPD_PORT-${this_test#t}}
+test_set_port LIB_HTTPD_PORT
 
 TEST_PATH="$TEST_DIRECTORY"/lib-httpd
 HTTPD_ROOT_PATH="$PWD"/httpd
diff --git a/t/t0410-partial-clone.sh b/t/t0410-partial-clone.sh
index 169f7f10a7..bce02788e6 100755
--- a/t/t0410-partial-clone.sh
+++ b/t/t0410-partial-clone.sh
@@ -492,7 +492,6 @@ test_expect_success 'gc stops traversal when a missing but promised object is re
 	! grep "$TREE_HASH" out
 '
 
-LIB_HTTPD_PORT=12345  # default port, 410, cannot be used as non-root
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd
 
diff --git a/t/t5512-ls-remote.sh b/t/t5512-ls-remote.sh
index 32e722db2e..cd9e60632d 100755
--- a/t/t5512-ls-remote.sh
+++ b/t/t5512-ls-remote.sh
@@ -260,7 +260,7 @@ test_lazy_prereq GIT_DAEMON '
 # This test spawns a daemon, so run it only if the user would be OK with
 # testing with git-daemon.
 test_expect_success PIPE,JGIT,GIT_DAEMON 'indicate no refs in standards-compliant empty remote' '
-	JGIT_DAEMON_PORT=${JGIT_DAEMON_PORT-${this_test#t}} &&
+	test_set_port JGIT_DAEMON_PORT &&
 	JGIT_DAEMON_PID= &&
 	git init --bare empty.git &&
 	>empty.git/git-daemon-export-ok &&
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 6b3bbf99e4..92cf8f812c 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1263,3 +1263,42 @@ test_oid () {
 	fi &&
 	eval "printf '%s' \"\${$var}\""
 }
+
+# Choose a port number based on the test script's number and store it in
+# the given variable name, unless that variable already contains a number.
+test_set_port () {
+	local var=$1 port
+
+	if test $# -ne 1 || test -z "$var"
+	then
+		BUG "test_set_port requires a variable name"
+	fi
+
+	eval port=\$$var
+	case "$port" in
+	"")
+		# No port is set in the given env var, use the test
+		# number as port number instead.
+		# Remove not only the leading 't', but all leading zeros
+		# as well, so the arithmetic below won't (mis)interpret
+		# a test number like '0123' as an octal value.
+		port=${this_test#${this_test%%[1-9]*}}
+		if test "${port:-0}" -lt 1024
+		then
+			# root-only port, use a larger one instead.
+			port=$(($port + 10000))
+		fi
+		;;
+	*[^0-9]*|0*)
+		error >&7 "invalid port number: $port"
+		;;
+	*)
+		# The user has specified the port.
+		;;
+	esac
+
+	# Make sure that parallel '--stress' test jobs get different
+	# ports.
+	port=$(($port + ${GIT_TEST_STRESS_JOB_NR:-0}))
+	eval $var=$port
+}
diff --git a/t/test-lib.sh b/t/test-lib.sh
index c34831a4de..a1abb1177a 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -71,19 +71,222 @@ then
 	exit 1
 fi
 
+# Parse options while taking care to leave $@ intact, so we will still
+# have all the original command line options when executing the test
+# script again for '--tee' and '--verbose-log' below.
+store_arg_to=
+prev_opt=
+for opt
+do
+	if test -n "$store_arg_to"
+	then
+		eval $store_arg_to=\$opt
+		store_arg_to=
+		prev_opt=
+		continue
+	fi
+
+	case "$opt" in
+	-d|--d|--de|--deb|--debu|--debug)
+		debug=t ;;
+	-i|--i|--im|--imm|--imme|--immed|--immedi|--immedia|--immediat|--immediate)
+		immediate=t ;;
+	-l|--l|--lo|--lon|--long|--long-|--long-t|--long-te|--long-tes|--long-test|--long-tests)
+		GIT_TEST_LONG=t; export GIT_TEST_LONG ;;
+	-r)
+		store_arg_to=run_list
+		;;
+	--run=*)
+		run_list=${opt#--*=} ;;
+	-h|--h|--he|--hel|--help)
+		help=t ;;
+	-v|--v|--ve|--ver|--verb|--verbo|--verbos|--verbose)
+		verbose=t ;;
+	--verbose-only=*)
+		verbose_only=${opt#--*=}
+		;;
+	-q|--q|--qu|--qui|--quie|--quiet)
+		# Ignore --quiet under a TAP::Harness. Saying how many tests
+		# passed without the ok/not ok details is always an error.
+		test -z "$HARNESS_ACTIVE" && quiet=t ;;
+	--with-dashes)
+		with_dashes=t ;;
+	--no-color)
+		color= ;;
+	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind)
+		valgrind=memcheck
+		tee=t
+		;;
+	--valgrind=*)
+		valgrind=${opt#--*=}
+		tee=t
+		;;
+	--valgrind-only=*)
+		valgrind_only=${opt#--*=}
+		tee=t
+		;;
+	--tee)
+		tee=t ;;
+	--root=*)
+		root=${opt#--*=} ;;
+	--chain-lint)
+		GIT_TEST_CHAIN_LINT=1 ;;
+	--no-chain-lint)
+		GIT_TEST_CHAIN_LINT=0 ;;
+	-x)
+		trace=t ;;
+	-V|--verbose-log)
+		verbose_log=t
+		tee=t
+		;;
+	--stress)
+		stress=t ;;
+	--stress=*)
+		stress=${opt#--*=}
+		case "$stress" in
+		*[^0-9]*|0*|"")
+			echo "error: --stress=<N> requires the number of jobs to run" >&2
+			exit 1
+			;;
+		*)	# Good.
+			;;
+		esac
+		;;
+	*)
+		echo "error: unknown test option '$opt'" >&2; exit 1 ;;
+	esac
+
+	prev_opt=$opt
+done
+if test -n "$store_arg_to"
+then
+	echo "error: $prev_opt requires an argument" >&2
+	exit 1
+fi
+
+if test -n "$valgrind_only"
+then
+	test -z "$valgrind" && valgrind=memcheck
+	test -z "$verbose" && verbose_only="$valgrind_only"
+elif test -n "$valgrind"
+then
+	test -z "$verbose_log" && verbose=t
+fi
+
+if test -n "$stress"
+then
+	verbose=t
+	trace=t
+	immediate=t
+fi
+
+TEST_STRESS_JOB_SFX="${GIT_TEST_STRESS_JOB_NR:+.stress-$GIT_TEST_STRESS_JOB_NR}"
+TEST_NAME="$(basename "$0" .sh)"
+TEST_RESULTS_DIR="$TEST_OUTPUT_DIRECTORY/test-results"
+TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME$TEST_STRESS_JOB_SFX"
+TRASH_DIRECTORY="trash directory.$TEST_NAME$TEST_STRESS_JOB_SFX"
+test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
+case "$TRASH_DIRECTORY" in
+/*) ;; # absolute path is good
+ *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
+esac
+
+# If --stress was passed, run this test repeatedly in several parallel loops.
+if test "$GIT_TEST_STRESS_STARTED" = "done"
+then
+	: # Don't stress test again.
+elif test -n "$stress"
+then
+	if test "$stress" != t
+	then
+		job_count=$stress
+	elif test -n "$GIT_TEST_STRESS_LOAD"
+	then
+		job_count="$GIT_TEST_STRESS_LOAD"
+	elif job_count=$(getconf _NPROCESSORS_ONLN 2>/dev/null) &&
+	     test -n "$job_count"
+	then
+		job_count=$((2 * $job_count))
+	else
+		job_count=8
+	fi
+
+	mkdir -p "$TEST_RESULTS_DIR"
+	stressfail="$TEST_RESULTS_BASE.stress-failed"
+	rm -f "$stressfail"
+
+	stress_exit=0
+	trap '
+		kill $job_pids 2>/dev/null
+		wait
+		stress_exit=1
+	' TERM INT HUP
+
+	job_pids=
+	job_nr=0
+	while test $job_nr -lt "$job_count"
+	do
+		(
+			GIT_TEST_STRESS_STARTED=done
+			GIT_TEST_STRESS_JOB_NR=$job_nr
+			export GIT_TEST_STRESS_STARTED GIT_TEST_STRESS_JOB_NR
+
+			trap '
+				kill $test_pid 2>/dev/null
+				wait
+				exit 1
+			' TERM INT
+
+			cnt=0
+			while ! test -e "$stressfail"
+			do
+				$TEST_SHELL_PATH "$0" "$@" >"$TEST_RESULTS_BASE.stress-$job_nr.out" 2>&1 &
+				test_pid=$!
+
+				if wait $test_pid
+				then
+					printf "OK   %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
+				else
+					echo $GIT_TEST_STRESS_JOB_NR >>"$stressfail"
+					printf "FAIL %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
+				fi
+				cnt=$(($cnt + 1))
+			done
+		) &
+		job_pids="$job_pids $!"
+		job_nr=$(($job_nr + 1))
+	done
+
+	wait
+
+	if test -f "$stressfail"
+	then
+		echo "Log(s) of failed test run(s):"
+		for failed_job_nr in $(sort -n "$stressfail")
+		do
+			echo "Contents of '$TEST_RESULTS_BASE.stress-$failed_job_nr.out':"
+			cat "$TEST_RESULTS_BASE.stress-$failed_job_nr.out"
+		done
+		rm -rf "$TRASH_DIRECTORY.stress-failed"
+		# Move the last one.
+		mv "$TRASH_DIRECTORY.stress-$failed_job_nr" "$TRASH_DIRECTORY.stress-failed"
+	fi
+
+	exit $stress_exit
+fi
+
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
-case "$GIT_TEST_TEE_STARTED, $* " in
-done,*)
-	# do not redirect again
-	;;
-*' --tee '*|*' --va'*|*' -V '*|*' --verbose-log '*)
-	mkdir -p "$TEST_OUTPUT_DIRECTORY/test-results"
-	BASE="$TEST_OUTPUT_DIRECTORY/test-results/$(basename "$0" .sh)"
+if test "$GIT_TEST_TEE_STARTED" = "done"
+then
+	: # do not redirect again
+elif test -n "$tee"
+then
+	mkdir -p "$TEST_RESULTS_DIR"
 
 	# Make this filename available to the sub-process in case it is using
 	# --verbose-log.
-	GIT_TEST_TEE_OUTPUT_FILE=$BASE.out
+	GIT_TEST_TEE_OUTPUT_FILE=$TEST_RESULTS_BASE.out
 	export GIT_TEST_TEE_OUTPUT_FILE
 
 	# Truncate before calling "tee -a" to get rid of the results
@@ -91,11 +294,38 @@ done,*)
 	>"$GIT_TEST_TEE_OUTPUT_FILE"
 
 	(GIT_TEST_TEE_STARTED=done ${TEST_SHELL_PATH} "$0" "$@" 2>&1;
-	 echo $? >"$BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
-	test "$(cat "$BASE.exit")" = 0
+	 echo $? >"$TEST_RESULTS_BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
+	test "$(cat "$TEST_RESULTS_BASE.exit")" = 0
 	exit
-	;;
-esac
+fi
+
+if test -n "$trace" && test -n "$test_untraceable"
+then
+	# '-x' tracing requested, but this test script can't be reliably
+	# traced, unless it is run with a Bash version supporting
+	# BASH_XTRACEFD (introduced in Bash v4.1).
+	#
+	# Perform this version check _after_ the test script was
+	# potentially re-executed with $TEST_SHELL_PATH for '--tee' or
+	# '--verbose-log', so the right shell is checked and the
+	# warning is issued only once.
+	if test -n "$BASH_VERSION" && eval '
+	     test ${BASH_VERSINFO[0]} -gt 4 || {
+	       test ${BASH_VERSINFO[0]} -eq 4 &&
+	       test ${BASH_VERSINFO[1]} -ge 1
+	     }
+	   '
+	then
+		: Executed by a Bash version supporting BASH_XTRACEFD.  Good.
+	else
+		echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
+		trace=
+	fi
+fi
+if test -n "$trace" && test -z "$verbose_log"
+then
+	verbose=t
+fi
 
 # For repeatability, reset the environment to known value.
 # TERM is sanitized below, after saving color control sequences.
@@ -193,7 +423,7 @@ fi
 
 # Add libc MALLOC and MALLOC_PERTURB test
 # only if we are not executing the test with valgrind
-if expr " $GIT_TEST_OPTS " : ".* --valgrind " >/dev/null ||
+if test -n "$valgrind" ||
    test -n "$TEST_NO_MALLOC_CHECK"
 then
 	setup_malloc_check () {
@@ -264,100 +494,6 @@ test "x$TERM" != "xdumb" && (
 	) &&
 	color=t
 
-while test "$#" -ne 0
-do
-	case "$1" in
-	-d|--d|--de|--deb|--debu|--debug)
-		debug=t; shift ;;
-	-i|--i|--im|--imm|--imme|--immed|--immedi|--immedia|--immediat|--immediate)
-		immediate=t; shift ;;
-	-l|--l|--lo|--lon|--long|--long-|--long-t|--long-te|--long-tes|--long-test|--long-tests)
-		GIT_TEST_LONG=t; export GIT_TEST_LONG; shift ;;
-	-r)
-		shift; test "$#" -ne 0 || {
-			echo 'error: -r requires an argument' >&2;
-			exit 1;
-		}
-		run_list=$1; shift ;;
-	--run=*)
-		run_list=${1#--*=}; shift ;;
-	-h|--h|--he|--hel|--help)
-		help=t; shift ;;
-	-v|--v|--ve|--ver|--verb|--verbo|--verbos|--verbose)
-		verbose=t; shift ;;
-	--verbose-only=*)
-		verbose_only=${1#--*=}
-		shift ;;
-	-q|--q|--qu|--qui|--quie|--quiet)
-		# Ignore --quiet under a TAP::Harness. Saying how many tests
-		# passed without the ok/not ok details is always an error.
-		test -z "$HARNESS_ACTIVE" && quiet=t; shift ;;
-	--with-dashes)
-		with_dashes=t; shift ;;
-	--no-color)
-		color=; shift ;;
-	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind)
-		valgrind=memcheck
-		shift ;;
-	--valgrind=*)
-		valgrind=${1#--*=}
-		shift ;;
-	--valgrind-only=*)
-		valgrind_only=${1#--*=}
-		shift ;;
-	--tee)
-		shift ;; # was handled already
-	--root=*)
-		root=${1#--*=}
-		shift ;;
-	--chain-lint)
-		GIT_TEST_CHAIN_LINT=1
-		shift ;;
-	--no-chain-lint)
-		GIT_TEST_CHAIN_LINT=0
-		shift ;;
-	-x)
-		# Some test scripts can't be reliably traced  with '-x',
-		# unless the test is run with a Bash version supporting
-		# BASH_XTRACEFD (introduced in Bash v4.1).  Check whether
-		# this test is marked as such, and ignore '-x' if it
-		# isn't executed with a suitable Bash version.
-		if test -z "$test_untraceable" || {
-		     test -n "$BASH_VERSION" && eval '
-		       test ${BASH_VERSINFO[0]} -gt 4 || {
-			 test ${BASH_VERSINFO[0]} -eq 4 &&
-			 test ${BASH_VERSINFO[1]} -ge 1
-		       }
-		     '
-		   }
-		then
-			trace=t
-		else
-			echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
-		fi
-		shift ;;
-	-V|--verbose-log)
-		verbose_log=t
-		shift ;;
-	*)
-		echo "error: unknown test option '$1'" >&2; exit 1 ;;
-	esac
-done
-
-if test -n "$valgrind_only"
-then
-	test -z "$valgrind" && valgrind=memcheck
-	test -z "$verbose" && verbose_only="$valgrind_only"
-elif test -n "$valgrind"
-then
-	test -z "$verbose_log" && verbose=t
-fi
-
-if test -n "$trace" && test -z "$verbose_log"
-then
-	verbose=t
-fi
-
 if test -n "$color"
 then
 	# Save the color control sequences now rather than run tput
@@ -476,7 +612,7 @@ die () {
 
 GIT_EXIT_OK=
 trap 'die' EXIT
-trap 'exit $?' INT
+trap 'exit $?' INT TERM HUP
 
 # The user-facing functions are loaded from a separate file so that
 # test_perf subshells can have them too
@@ -818,12 +954,9 @@ test_done () {
 
 	if test -z "$HARNESS_ACTIVE"
 	then
-		test_results_dir="$TEST_OUTPUT_DIRECTORY/test-results"
-		mkdir -p "$test_results_dir"
-		base=${0##*/}
-		test_results_path="$test_results_dir/${base%.sh}.counts"
+		mkdir -p "$TEST_RESULTS_DIR"
 
-		cat >"$test_results_path" <<-EOF
+		cat >"$TEST_RESULTS_BASE.counts" <<-EOF
 		total $test_count
 		success $test_success
 		fixed $test_fixed
@@ -1029,12 +1162,6 @@ then
 fi
 
 # Test repository
-TRASH_DIRECTORY="trash directory.$(basename "$0" .sh)"
-test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
-case "$TRASH_DIRECTORY" in
-/*) ;; # absolute path is good
- *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
-esac
 rm -fr "$TRASH_DIRECTORY" || {
 	GIT_EXIT_OK=t
 	echo >&5 "FATAL: Cannot prepare test area"