fix the compose call: it never worked outside --dry-run

Setting IFS to a newline to pass the environment also stopped the command
line from splitting, so '-f a -f b' went out as a single argument and the two
words 'docker compose' were looked up as one program name. Every real command
was broken; only --dry-run, which prints a string, looked right.

The variables are now exported in a subshell instead of going through env(1),
which keeps values with spaces intact and leaves the command line to normal
word splitting.

The tests only checked what --dry-run printed, which is why they missed it.
spec/unit/compose_spec.sh now asserts the argv that is actually executed, for
both the plugin and the standalone binary.

spec/support/bin/docker

@@ -1,6 +1,10 @@
 #!/bin/sh
 # Mock docker for myos tests: logs every call, answers a few read-only queries deterministically.
-[ -n "${MYOS_DOCKER_LOG:-}" ] && printf 'docker %s\n' "$*" >> "$MYOS_DOCKER_LOG"
+if [ -n "${MYOS_DOCKER_LOG:-}" ]; then
+  printf 'docker %s\n' "$*" >> "$MYOS_DOCKER_LOG"
+  # the environment is recorded too, so a test can assert what was exported
+  env > "$(dirname "$MYOS_DOCKER_LOG")/env.log"
+fi
 case "$1 $2" in
   "compose version") echo "${MOCK_COMPOSE_VERSION:-2.29.0}" ;;
   "compose config")  echo "# mock compose config" ;;

spec/support/bin/docker-compose

@@ -1,4 +1,8 @@
 #!/bin/sh
-[ -n "${MYOS_DOCKER_LOG:-}" ] && printf 'docker-compose %s\n' "$*" >> "$MYOS_DOCKER_LOG"
+if [ -n "${MYOS_DOCKER_LOG:-}" ]; then
+  printf 'docker-compose %s\n' "$*" >> "$MYOS_DOCKER_LOG"
+  # the environment is recorded too, so a test can assert what was exported
+  env > "$(dirname "$MYOS_DOCKER_LOG")/env.log"
+fi
 case "$1" in version) echo "${MOCK_COMPOSE_VERSION:-2.29.0}" ;; config) echo "# mock compose config" ;; *) : ;; esac
 exit 0

spec/unit/compose_spec.sh

@@ -0,0 +1,75 @@
+#shellcheck shell=sh
+Include lib/str.sh
+Include lib/core.sh
+Include lib/tags.sh
+Include lib/config.sh
+Include lib/compose.sh
+
+# These assertions look at what is actually executed, not at what --dry-run
+# prints: the two used to disagree, because the execution path set IFS to a
+# newline and so never split "-f a -f b", nor the two words "docker compose".
+Describe 'lib/compose.sh execution'
+  setup() {
+    MYOS_TMP=$(mktemp -d "${TMPDIR:-/tmp}/myos-compose.XXXXXX")
+    MYOS_TMP=$(cd "$MYOS_TMP" && pwd -P)
+    printf 'services:\n  a:\n    image: ${IMAGE}\n' > "$MYOS_TMP/a.yml"
+    printf 'services:\n  b:\n    image: alpine\n' > "$MYOS_TMP/b.yml"
+    # exported, otherwise the mock (a child process) never sees it
+    export MYOS_DOCKER_LOG=$MYOS_TMP/docker.log
+    export PATH=$SPEC_DIR/support/bin:$PATH
+    DRYRUN=false
+    IMAGE=alpine:3.20
+    unset MYOS_COMPOSE_BIN 2>/dev/null || true
+  }
+  cleanup() { rm -rf "$MYOS_TMP"; }
+  BeforeEach setup
+  AfterEach cleanup
+
+  logged() { cat "$MYOS_DOCKER_LOG"; }
+
+  two_files() { myos_compose demo "$(printf '%s\n%s' "$MYOS_TMP/a.yml" "$MYOS_TMP/b.yml")" -- config; }
+
+  It 'passes every -f as its own argument'
+    When call two_files
+    The status should be success
+    The result of function logged should include "-f $MYOS_TMP/a.yml -f $MYOS_TMP/b.yml"
+    The result of function logged should include "-p demo"
+    The result of function logged should include "--project-directory $MYOS_TMP"
+    The result of function logged should end with "config"
+  End
+
+  It 'splits the two words of the docker compose plugin'
+    MYOS_COMPOSE_BIN="docker compose"
+    When call myos_compose demo "$MYOS_TMP/a.yml" -- config
+    The status should be success
+    The result of function logged should start with "docker compose --ansi=auto"
+  End
+
+  It 'runs the docker-compose binary as one word'
+    MYOS_COMPOSE_BIN="docker-compose"
+    When call myos_compose demo "$MYOS_TMP/a.yml" -- config
+    The status should be success
+    The result of function logged should start with "docker-compose --ansi=auto"
+  End
+
+  It 'passes the variables the compose files reference'
+    MYOS_COMPOSE_BIN="docker-compose"
+    When call myos_compose demo "$MYOS_TMP/a.yml" -- config
+    The status should be success
+    The contents of file "$MYOS_TMP/env.log" should include "IMAGE=alpine:3.20"
+  End
+
+  It 'keeps a value that contains spaces in one piece'
+    MYOS_COMPOSE_BIN="docker-compose"
+    IMAGE="alpine with spaces"
+    When call myos_compose demo "$MYOS_TMP/a.yml" -- config
+    The status should be success
+    The contents of file "$MYOS_TMP/env.log" should include "IMAGE=alpine with spaces"
+  End
+
+  It 'refuses to run without a compose file'
+    When call myos_compose demo "" -- config
+    The status should equal 3
+    The stderr should include "no compose file"
+  End
+End