Implement Multi-Lower-Layer Overlay Support through Merging (#122)

* feat: add -L flag to support one or more lower directories * docs: shell completion, manpage, and readme update * test: add test to test merging multiple lowerdirs, and also invoking -L multiple times Note that currently -L implies -n (will not commit changes), support for that is being tracked in #142 --------- Co-authored-by: gliargovas <[email protected]> Co-authored-by: Ezri Zhu <[email protected]> Reviewed-by: Michael Greenberg <[email protected]> Reviewed-by: Konstantinos Kallas <[email protected]>
binpash · Jan 13, 2024 · 0647f69 · 0647f69
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diff --git a/README.md b/README.md
@@ -156,6 +156,22 @@ $ try commit rustup-sandbox
 You can also run `try explore` to open your current shell in try, or `/try
 explore /tmp/tmp.X6OQb5tJwr` to explore an existing sandbox.
 
+To specify multiple lower directories for overlay (by merging them together), you can use the `-L` (implies `-n`) flag followed by a colon-separated list of directories. The directories on the left have higher precedence and can overwrite the directories on the right:
+
+```ShellSession
+$ try -D /tmp/sandbox1 "echo 'File 1 Contents - sandbox1' > file1.txt"
+$ try -D /tmp/sandbox2 "echo 'File 2 Contents - sandbox2' > file2.txt"
+$ try -D /tmp/sandbox3 "echo 'File 2 Contents - sandbox3' > file2.txt"
+
+# Now use the -L flag to merge both sandbox directories together, with sandbox3 having precedence over sandbox2
+$ try -L "/tmp/sandbox3:/tmp/sandbox2:/tmp/sandbox1" "cat file1.txt file2.txt"
+File 1 Contents - sandbox1
+File 2 Contents - sandbox3
+```
+
+In this example, `try` will merge `/sandbox1`, `/sandbox2` and `/sandbox3` together before mounting the overlay. This way, you can combine the contents of multiple `try` sandboxes.
+
+
 ## Known Issues
 Any command that interacts with other users/groups will fail since only the
 current user's UID/GID are mapped. However, the [future

diff --git a/completions/try.bash b/completions/try.bash
@@ -17,13 +17,16 @@ _try() {
 
     case "${cmd}" in
         (try)
-            opts="-n -y -v -h -x -i -D -U summary commit explore"
+            opts="-n -y -v -h -x -i -D -U -L summary commit explore"
             if [[ ${cur} == -* || ${COMP_CWORD} -eq 1 ]]
             then
                 COMPREPLY=( $(compgen -W "${opts}" -- "${cur}") )
                 return 0
             fi
             case "${prev}" in
+                (-L)
+                    COMPREPLY=($(compgen -d "${cur}"))
+                    return 0;;
                 (-D)
                     COMPREPLY=($(compgen -d "${cur}"))
                     return 0;;

diff --git a/docs/try.1.md b/docs/try.1.md
@@ -6,7 +6,7 @@
 try - run a command in an overlay
 
 # SYNOPSIS
-| try [-ny] [-i PATTERN] [-D DIR] [-U PATH] CMD [ARG ...]
+| try [-ny] [-i PATTERN] [-D DIR] [-U PATH] [-L LOWER_DIRS] CMD [ARG ...]
 | try summary [DIR]
 | try commit [DIR]
 | try explore
@@ -57,6 +57,11 @@ While using *try* you can choose to commit the result to the filesystem or compl
 : Use the unionfs helper implementation defined in the *PATH* (e.g., mergerfs, unionfs-fuse) instead of the default.
 This option is recommended in case OverlayFS fails.
 
+-L *LOWER_DIRS*
+
+: Specify a colon-separated list of directories to be used as lower directories for the overlay, formatted as "dir1:dir2:...:dirn" (implies -n).
+
+
 ## Subcommands
 
 try summary *DIR*
@@ -128,6 +133,12 @@ Alternatively, you can specify your own overlay directory as follows (note that
 try -D try_dir gunzip file.txt.gz
 ```
 
+To use multiple lower directories for overlay (by merging them), you can use the `-L` flag followed by a colon-separated list of directories. The directories on the left have higher precedence and can overwrite the directories on the right:
+
+```
+try -L /lowerdir1:/lowerdir2:/lowerdir3 gunzip file.txt.gz
+```
+
 You can inspect the changes made inside a given overlay directory:
 
 ```

diff --git a/test/merge_multiple_dirs.sh b/test/merge_multiple_dirs.sh
@@ -0,0 +1,71 @@
+#!/bin/sh
+
+TRY_TOP="${TRY_TOP:-$(git rev-parse --show-toplevel --show-superproject-working-tree)}"
+TRY="$TRY_TOP/try"
+
+cleanup() {
+    cd /
+
+    if [ -d "$try_workspace" ]
+    then
+        rm -rf "$try_workspace" >/dev/null 2>&1
+    fi
+
+    if [ -f "$try_example_dir1" ]
+    then
+        rm "$try_example_dir1"
+    fi
+
+    if [ -f "$try_example_dir2" ]
+    then
+        rm "$try_example_dir2"
+    fi
+
+    if [ -f "$try_example_dir3" ]
+    then
+        rm "$try_example_dir3"
+    fi
+
+    if [ -f "$expected1" ]
+    then
+       rm "$expected1"
+    fi
+
+    if [ -f "$expected2" ]
+    then
+       rm "$expected2"
+    fi
+    if [ -f "$expected3" ]
+    then
+       rm "$expected3"
+    fi
+}
+
+trap 'cleanup' EXIT
+
+try_workspace="$(mktemp -d -p .)"
+cp "$TRY_TOP/test/resources/file.txt.gz" "$try_workspace/"
+cd "$try_workspace" || return 1
+
+try_example_dir1="$(mktemp -d)"
+try_example_dir2="$(mktemp -d)"
+try_example_dir3="$(mktemp -d)"
+
+expected1="$(mktemp)"
+expected2="$(mktemp)"
+expected3="$(mktemp)"
+
+touch "$expected1"
+echo "test2" > "$expected2"
+echo "test3" > "$expected3"
+
+"$TRY" -D "$try_example_dir1" "touch file_1.txt; echo test > file_2.txt; rm file.txt.gz" || return 2
+"$TRY" -D "$try_example_dir2" "echo test2 > file_2.txt" || return 3
+"$TRY" -D "$try_example_dir3" "echo test3 > file_3.txt" || return 4
+"$TRY" -L "$try_example_dir3:$try_example_dir2:$try_example_dir1" -y "cat file_1.txt > out1; cat file_2.txt > out2; cat file_3.txt > out3"|| return 5
+
+diff -q "$expected1" out1 || return 6
+diff -q "$expected2" out2 || return 7
+diff -q "$expected3" out3 || return 8
+
+! [ -f out4 ] || return 9
diff --git a/try b/try
@@ -45,7 +45,7 @@ try() {
     fi
 
     ## Make any directories that don't already exist, this is OK to do here
-    ##  because we have already checked if it valid.
+    ## because we have already checked if it valid.
     export SANDBOX_DIR
     mkdir -p "$SANDBOX_DIR/upperdir" "$SANDBOX_DIR/workdir" "$SANDBOX_DIR/temproot"
 
@@ -56,6 +56,34 @@ try() {
     findmnt --real -r -o target -n >>"$DIRS_AND_MOUNTS"
     sort -u -o "$DIRS_AND_MOUNTS" "$DIRS_AND_MOUNTS"
 
+    # Calculate UPDATED_DIRS_AND_MOUNTS that contains the merge arguments in LOWER_DIRS
+    UPDATED_DIRS_AND_MOUNTS="$(mktemp)"
+    export UPDATED_DIRS_AND_MOUNTS
+    while IFS="" read -r mountpoint
+    do
+        new_mountpoint=""
+        OLDIFS=$IFS
+        IFS=":"
+
+        for lower_dir in $LOWER_DIRS
+        do
+            temp_mountpoint="$lower_dir/upperdir$mountpoint"
+            if [ -n "$new_mountpoint" ]
+            then
+                # If new_mountpoint is not empty, append : and the temp_mountpoint
+                new_mountpoint="$new_mountpoint:$temp_mountpoint"
+            else
+                # If new_mountpoint is empty, just set it to temp_mountpoint
+                new_mountpoint="$temp_mountpoint"
+            fi
+        done
+        IFS=$OLDIFS
+        # Add the original mountpoint at the end
+        new_mountpoint="${new_mountpoint:+$new_mountpoint:}$mountpoint"
+        echo "$new_mountpoint" >> "$UPDATED_DIRS_AND_MOUNTS"
+    done <"$DIRS_AND_MOUNTS"
+
+
     # we will overlay-mount each root directory separately (instead of all at once) because some directories cannot be overlayed
     # so we set up the mount points now
     #
@@ -94,11 +122,12 @@ TRY_COMMAND="$TRY_COMMAND($0)"
 ## A wrapper of `mount -t overlay` to have cleaner looking code
 make_overlay() {
     sandbox_dir="$1"
-    lowerdir="$2"
-    mountpoint="$3"
-    mount -t overlay overlay -o userxattr -o "lowerdir=$lowerdir,upperdir=$sandbox_dir/upperdir/$mountpoint,workdir=$sandbox_dir/workdir/$mountpoint" "$sandbox_dir/temproot/$mountpoint"
+    lowerdirs="$2"
+    overlay_mountpoint="$3"
+    mount -t overlay overlay -o userxattr -o "lowerdir=$lowerdirs,upperdir=$sandbox_dir/upperdir/$overlay_mountpoint,workdir=$sandbox_dir/workdir/$overlay_mountpoint" "$sandbox_dir/temproot/$overlay_mountpoint"
 }
 
+
 devices_to_mount="tty null zero full random urandom"
 
 ## Mounts and unmounts a few select devices instead of the whole `/dev`
@@ -138,24 +167,23 @@ then
 fi
 
 # actually mount the overlays
-for mountpoint in $(cat "$DIRS_AND_MOUNTS")
+for mountpoint in $(cat "$UPDATED_DIRS_AND_MOUNTS")
 do
+    pure_mountpoint=${mountpoint##*:}
+
     ## We are not interested in mounts that are not directories
-    if ! [ -d "$mountpoint" ]
+    if ! [ -d "$pure_mountpoint" ]
     then
         continue
     fi
 
-    ## Don't do anything for the root
-    ##   and skip if it is /dev or /proc, we will mount it later
-    if [ "$mountpoint" = "/" ] ||
-        [ "$mountpoint" = "/dev" ] || [ "$mountpoint" = "/proc" ]
-    then
-        continue
-    fi
+    ## Don't do anything for the root and skip if it is /dev or /proc, we will mount it later
+    case "$pure_mountpoint" in
+        (/|/dev|/proc) continue;;
+    esac
 
     # Try mounting everything normally
-    make_overlay "$SANDBOX_DIR" "/$mountpoint" "$mountpoint" 2>>"$try_mount_log"
+    make_overlay "$SANDBOX_DIR" "$mountpoint" "$pure_mountpoint" 2>>"$try_mount_log"
     # If mounting everything normally fails, we try using either using mergerfs or unionfs to mount them.
     if [ "$?" -ne 0 ]
     then
@@ -180,9 +208,7 @@ do
             ## Create a union directory
             "$UNION_HELPER" $mountpoint $merger_dir 2>>"$try_mount_log" ||
                 printf "%s: Warning: Failed mounting $mountpoint via $UNION_HELPER, see \"$try_mount_log\"\n" "$TRY_COMMAND" >&2
-
-            ## Make the overlay on the union directory which works as a lowerdir for overlay
-            make_overlay "$SANDBOX_DIR" "$merger_dir" "$mountpoint" 2>>"$try_mount_log" ||
+            make_overlay "$SANDBOX_DIR" "$merger_dir" "$pure_mountpoint" 2>>"$try_mount_log" ||
             printf "%s: Warning: Failed mounting $mountpoint as an overlay via $UNION_HELPER, see \"$try_mount_log\"\n" "$TRY_COMMAND" >&2
         fi
     fi
@@ -477,19 +503,18 @@ error() {
 
 usage() {
     cat >&2 <<EOF
-Usage: $TRY_COMMAND [-nvhyx] [-i PATTERN] [-D DIR] [-U PATH] CMD [ARG ...]
+Usage: $TRY_COMMAND [-nvhyx] [-i PATTERN] [-D DIR] [-U PATH] [-L dir1:dir2:...] CMD [ARG ...]
 
   -n                don't commit or prompt for commit (overrides -y)
   -y                assume yes to all prompts (overrides -n)
   -x                prevent network access (by unsharing the network namespace)
   -i PATTERN        ignore paths that match PATTERN on summary and commit
   -D DIR            work in DIR (implies -n)
   -U PATH           path to unionfs helper (e.g., mergerfs, unionfs-fuse)
-
+  -L dir1:dir2:...  specify multiple lower directories to merge (colon-separated, implies -n)
   -v                show version information (and exit)
   -h                show this usage message (and exit)
 
-
 Subcommands:
   $TRY_COMMAND summary DIR   show the summary for the overlay in DIR
   $TRY_COMMAND commit DIR    commit the overlay in DIR
@@ -510,7 +535,8 @@ NO_COMMIT="interactive"
 # Includes all patterns given using the `-i` flag; will be used with `grep -f`
 IGNORE_FILE="$(mktemp)"
 
-while getopts ":yvnhxi:D:U:" opt
+while getopts ":yvnhxi:D:U:L:" opt
+
 do
     case "$opt" in
         (y)   NO_COMMIT="commit";;
@@ -522,6 +548,12 @@ do
               fi
               SANDBOX_DIR="$OPTARG"
               NO_COMMIT="quiet";;
+        (L)   if [ -n "$LOWER_DIRS" ]
+              then
+                  error "the -L option has been specified multiple times" 2
+              fi
+              LOWER_DIRS="$OPTARG"
+              NO_COMMIT="quiet";;
         (v)   echo "$TRY_COMMAND version $TRY_VERSION" >&2
               exit 0;;
         (U)   if ! [ -x "$OPTARG" ]