lib/repo: Don't syncfs or fsync() dirs if fsync opt is disabled

There are use cases for not syncing at all; think build cache repos, etc. Let's
be consistent here and make sure if fsync is disabled we do no sync at all.

I chose this opportunity to add tests using the shiny new strace fault
injection.  I can forsee using this for a lot more things, so I made
the support for detecting things generic.

Related: https://github.com/ostreedev/ostree/issues/1184

Closes: #1186
Approved by: jlebon

tests/basic-test.sh

@@ -19,7 +19,7 @@
 
 set -euo pipefail
 
-echo "1..$((72 + ${extra_basic_tests:-0}))"
+echo "1..$((73 + ${extra_basic_tests:-0}))"
 
 $CMD_PREFIX ostree --version > version.yaml
 python -c 'import yaml; yaml.safe_load(open("version.yaml"))'
@@ -809,8 +809,18 @@ cd ${test_tmpdir}
 rm -rf test2-checkout
 mkdir -p test2-checkout
 cd test2-checkout
-touch should-not-be-fsynced
-$OSTREE commit ${COMMIT_ARGS} -b test2 -s "Unfsynced commit" --fsync=false
+echo 'should not be fsynced' > should-not-be-fsynced
+if ! skip_one_without_strace_fault_injection; then
+    # Test that --fsync=false doesn't fsync
+    fsync_inject_error_ostree="strace -o /dev/null -f -e inject=syncfs,fsync,sync:error=EPERM ostree"
+    ${fsync_inject_error_ostree} --repo=${test_tmpdir}/repo commit ${COMMIT_ARGS} -b test2-no-fsync --fsync=false
+    # And test that we get EPERM if we inject an error
+    if ${fsync_inject_error_ostree} --repo=${test_tmpdir}/repo commit ${COMMIT_ARGS} -b test2-no-fsync 2>err.txt; then
+        fatal "fsync error injection failed"
+    fi
+    assert_file_has_content err.txt 'sync.*Operation not permitted'
+    echo "ok fsync disabled"
+fi
 
 # Run this test only as non-root user.  When run as root, the chmod
 # won't have any effect.

tests/libtest.sh

@@ -543,6 +543,29 @@ skip_without_user_xattrs () {
     fi
 }
 
+# https://brokenpi.pe/tools/strace-fault-injection
+_have_strace_fault_injection=''
+have_strace_fault_injection() {
+    if test "${_have_strace_fault_injection}" = ''; then
+        if strace -P ${test_srcdir}/libtest-core.sh -e inject=read:retval=0 cat ${test_srcdir}/libtest-core.sh >out.txt &&
+           test '!' -s out.txt; then
+            _have_strace_fault_injection=yes
+        else
+            _have_strace_fault_injection=no
+        fi
+        rm -f out.txt
+    fi
+    test ${_have_strace_fault_injection} = yes
+}
+
+skip_one_without_strace_fault_injection() {
+    if ! have_strace_fault_injection; then
+        echo "ok # SKIP this test requires strace fault injection"
+        return 0
+    fi
+    return 1
+}
+
 skip_without_fuse () {
     fusermount --version >/dev/null 2>&1 || skip "no fusermount"