etc-merge: Handle unmergable paths properly

We would have unmergable paths if a modified file was changed to a
directory in the new etc or vice-versa. One of the major issues was that
we were erroring out during "merge" which is not ideal. A new vector now
keeps track of all unmergable paths and the reason they're not mergable.

Ref: composefs/composefs-rs/issues/335

Signed-off-by: Pragyan Poudyal <pragyanpoudyal41999@gmail.com>

Author: Johan-Liebert1

crates/etc-merge/src/lib.rs

@@ -78,6 +78,12 @@ fn stat_eq_ignore_mtime(this: &Stat, other: &Stat) -> bool {
     return true;
 }
 
+#[derive(Debug)]
+pub struct UnmergablePaths {
+    path: PathBuf,
+    reason: String,
+}
+
 /// Represents the differences between two directory trees.
 #[derive(Debug)]
 pub struct Diff {
@@ -88,6 +94,8 @@ pub struct Diff {
     modified: Vec<PathBuf>,
     /// Paths that exist in the pristine /etc but not in the current one
     removed: Vec<PathBuf>,
+    /// Paths that are unmergable
+    unmergable_paths: Vec<UnmergablePaths>,
 }
 
 fn collect_all_files(
@@ -170,6 +178,61 @@ fn get_deletions(
     Ok(())
 }
 
+fn check_if_mergable(
+    new: &Directory<CustomMetadata>,
+    current_inode: &Inode<CustomMetadata>,
+    current_path: &Path,
+    diff: &mut Diff,
+) -> anyhow::Result<()> {
+    match current_inode {
+        // If currently 'file' is a directory, make sure it's not a regular file
+        // new_etc as well, else we can't merge
+        Inode::Directory(..) => {
+            let new_dir = new.get_directory(&current_path.as_os_str());
+
+            match new_dir {
+                Ok(_) => {}
+                Err(e) => match e {
+                    ImageError::NotADirectory(..) => {
+                        diff.unmergable_paths.push(UnmergablePaths {
+                            path: current_path.to_path_buf(),
+                            reason: format!(
+                                "Directory '{}' now defaults to a file in new etc",
+                                current_path.display()
+                            ),
+                        });
+                    }
+                    ImageError::NotFound(..) => {}
+
+                    _ => Err(e)?,
+                },
+            }
+        }
+
+        // If currently 'file' is not a directory, make sure it's not a directory in the
+        // new_etc either, else we can't merge
+        Inode::Leaf(..) => {
+            let new_dir = new.get_directory(&current_path.as_os_str());
+
+            match new_dir {
+                Ok(..) => {
+                    diff.unmergable_paths.push(UnmergablePaths {
+                        path: current_path.to_path_buf(),
+                        reason: format!(
+                            "File '{}' now defaults to a directory in new etc",
+                            current_path.display()
+                        ),
+                    });
+                }
+                Err(ImageError::NotFound(..)) => {},
+                Err(e) => Err(e)?,
+            }
+        }
+    }
+
+    Ok(())
+}
+
 // 1. Files in the currently booted deployment’s /etc which were modified from the default /usr/etc (of the same deployment) are retained.
 //
 // 2. Files in the currently booted deployment’s /etc which were not modified from the default /usr/etc (of the same deployment)
@@ -221,21 +284,39 @@ fn get_modifications(
                             diff,
                         )?;
 
-                        // This directory or its contents were modified/added
-                        // Check if the new directory was deleted from new_etc
-                        // If it was, we want to add the directory back
-                        if new.get_directory_opt(&current_path.as_os_str())?.is_none() {
-                            if diff.added.len() != total_added {
-                                diff.added.insert(total_added, current_path.clone());
-                            } else if diff.modified.len() != total_modified {
-                                diff.modified.insert(total_modified, current_path.clone());
+                        match new.get_directory(&current_path.as_os_str()) {
+                            Ok(..) => {
+                                // Directory exists in both current and new etc.
+                                // Modifications/additions within this directory are handled recursively.
+                                // No additional action needed here.
+                            }
+
+                            Err(ImageError::NotFound(..)) | Err(ImageError::NotADirectory(..)) => {
+                                // This directory was deleted in new_etc
+                                // If it was modified in the current etc, we want this back
+                                //
+                                // Was a directory in current etc, but is now
+                                // a file/symlink in the new etc
+                                if diff.added.len() != total_added {
+                                    diff.added.insert(total_added, current_path.clone());
+                                } else if diff.modified.len() != total_modified {
+                                    diff.modified.insert(total_modified, current_path.clone());
+                                }
                             }
+
+                            Err(e) => Err(e)?,
+                        }
+
+                        if diff.modified.len() != total_modified || diff.added.len() != total_added
+                        {
+                            check_if_mergable(new, inode, &current_path, diff)?;
                         }
                     }
 
                     Err(ImageError::NotFound(..)) => {
                         // Dir not found in original /etc, dir was added
                         diff.added.push(current_path.clone());
+                        check_if_mergable(new, inode, &current_path, diff)?;
 
                         // Also add every file inside that dir
                         collect_all_files(&curr_dir, current_path.clone(), &mut diff.added);
@@ -245,18 +326,21 @@ fn get_modifications(
                         // Some directory was changed to a file/symlink
                         // This should be counted in the diff, but we don't really merge this
                         diff.modified.push(current_path.clone());
+                        check_if_mergable(new, inode, &current_path, diff)?;
                     }
 
-                    Err(e) => Err(e)?,
+                    Err(e) => Err(e).with_context(|| format!("Opening pristine {path:?}"))?,
                 }
             }
 
             Inode::Leaf(leaf_id, _) => match pristine.leaf_id(path) {
                 Ok(old_leaf_id) => {
                     let leaf = &current_leaves[leaf_id.0];
                     let old_leaf = &pristine_leaves[old_leaf_id.0];
+
                     if !stat_eq_ignore_mtime(&old_leaf.stat, &leaf.stat) {
                         diff.modified.push(current_path.clone());
+                        check_if_mergable(new, inode, &current_path, diff)?;
                         current_path.pop();
                         continue;
                     }
@@ -266,19 +350,22 @@ fn get_modifications(
                             if old_meta.content_hash != current_meta.content_hash {
                                 // File modified in some way
                                 diff.modified.push(current_path.clone());
+                                check_if_mergable(new, inode, &current_path, diff)?;
                             }
                         }
 
                         (Symlink(old_link), Symlink(current_link)) => {
                             if old_link != current_link {
                                 // Symlink modified in some way
                                 diff.modified.push(current_path.clone());
+                                check_if_mergable(new, inode, &current_path, diff)?;
                             }
                         }
 
                         (Symlink(..), Regular(..)) | (Regular(..), Symlink(..)) => {
                             // File changed to symlink or vice-versa
                             diff.modified.push(current_path.clone());
+                            check_if_mergable(new, inode, &current_path, diff)?;
                         }
 
                         (a, b) => {
@@ -290,14 +377,16 @@ fn get_modifications(
                 Err(ImageError::IsADirectory(..)) => {
                     // A directory was changed to a file
                     diff.modified.push(current_path.clone());
+                    check_if_mergable(new, inode, &current_path, diff)?;
                 }
 
                 Err(ImageError::NotFound(..)) => {
                     // File not found in original /etc, file was added
                     diff.added.push(current_path.clone());
+                    check_if_mergable(new, inode, &current_path, diff)?;
                 }
 
-                Err(e) => Err(e).context(format!("{path:?}"))?,
+                Err(e) => Err(e).with_context(|| format!("Opening pristine {path:?}"))?,
             },
         }
 
@@ -385,6 +474,7 @@ pub fn compute_diff(
         added: vec![],
         modified: vec![],
         removed: vec![],
+        unmergable_paths: vec![],
     };
 
     get_modifications(
@@ -422,6 +512,15 @@ pub fn print_diff(diff: &Diff, writer: &mut impl Write) {
     for removed in &diff.removed {
         let _ = writeln!(writer, "{} {removed:?}", ModificationType::Removed.red());
     }
+
+    for unmergable in &diff.unmergable_paths {
+        let _ = writeln!(
+            writer,
+            "{} {}",
+            ModificationType::Unmergable.magenta(),
+            unmergable.reason
+        );
+    }
 }
 
 #[context("Collecting xattrs")]
@@ -589,6 +688,7 @@ enum ModificationType {
     Added,
     Modified,
     Removed,
+    Unmergable,
 }
 
 impl std::fmt::Display for ModificationType {
@@ -603,6 +703,7 @@ impl ModificationType {
             ModificationType::Added => "+",
             ModificationType::Modified => "~",
             ModificationType::Removed => "-",
+            ModificationType::Unmergable => "*",
         }
     }
 }
@@ -613,23 +714,35 @@ fn create_dir_with_perms(
     stat: &Stat,
     new_inode: Option<&Inode<CustomMetadata>>,
 ) -> anyhow::Result<()> {
-    // The new directory is not present in the new_etc, so we create it, else we only copy the
-    // metadata
-    if new_inode.is_none() {
-        // Here we use `create_dir_all` to create every parent as we will set the permissions later
-        // on. Due to the fact that we have an ordered (sorted) list of directories and directory
-        // entries and we have a DFS traversal, we will always have directory creation starting from
-        // the parent anyway.
-        //
-        // The exception being, if a directory is modified in the current_etc, and a new directory
-        // is added inside the modified directory, say `dir/prems` has its permissions modified and
-        // `dir/prems/new` is the new directory created. Since we handle added files/directories first,
-        // we will create the directories `perms/new` with directory `new` also getting its
-        // permissions set, but `perms` will not. `perms` will have its permissions set up when we
-        // handle the modified directories.
-        new_etc_fd
-            .create_dir_all(&dir_name)
-            .context(format!("Failed to create dir {dir_name:?}"))?;
+    match new_inode {
+        Some(inode) => match inode {
+            Inode::Directory(..) => { /* no-op */ }
+
+            Inode::Leaf(..) => {
+                anyhow::bail!(
+                    "Modified config directory {dir_name:?} newly defaults to file. Cannot merge"
+                )
+            }
+        },
+
+        // The new directory is not present in the new_etc, so we create it, else we only copy the
+        // metadata
+        None => {
+            // Here we use `create_dir_all` to create every parent as we will set the permissions later
+            // on. Due to the fact that we have an ordered (sorted) list of directories and directory
+            // entries and we have a DFS traversal, we will always have directory creation starting from
+            // the parent anyway.
+            //
+            // The exception being, if a directory is modified in the current_etc, and a new directory
+            // is added inside the modified directory, say `dir/prems` has its permissions modified and
+            // `dir/prems/new` is the new directory created. Since we handle added files/directories first,
+            // we will create the directories `perms/new` with directory `new` also getting its
+            // permissions set, but `perms` will not. `perms` will have its permissions set up when we
+            // handle the modified directories.
+            new_etc_fd
+                .create_dir_all(&dir_name)
+                .context(format!("Failed to create dir {dir_name:?}"))?;
+        }
     }
 
     new_etc_fd
@@ -730,12 +843,14 @@ fn merge_modified_files(
                             file,
                             current_inode.stat(current_leaves),
                             new_inode,
-                        )?;
+                        )
+                        .context("Merging directory")?;
                     }
 
                     Inode::Leaf(leaf_id, _) => {
                         let leaf = &current_leaves[leaf_id.0];
-                        merge_leaf(current_etc_fd, new_etc_fd, leaf, new_inode, file)?
+                        merge_leaf(current_etc_fd, new_etc_fd, leaf, new_inode, file)
+                            .context("Merging leaf")?
                     }
                 };
             }
@@ -755,7 +870,7 @@ fn merge_modified_files(
                 }
             },
 
-            Err(e) => Err(e)?,
+            Err(e) => Err(e).with_context(|| format!("Opening {file:?} in new etc"))?,
         };
     }