Added dry-run mode command line option (-n, --dry-run).

When invoked, Docuum will report what images would have been deleted
during it's initial vacuuming run at start up then exit.

In order to do this, Docuum now creates a list of images to delete
ensuring that by deleting them it will meet the space requirements set
forth by the user.

README.md

@@ -44,6 +44,9 @@ OPTIONS:
     -d, --deletion-chunk-size <DELETION CHUNK SIZE>
             Removes specified quantity of images at a time (default: 1)
 
+    -n, --dry-run
+            Dry run mode, prevents deletion of any images.
+
     -h, --help
             Prints help information
 

src/main.rs

@@ -35,6 +35,7 @@ const DEFAULT_THRESHOLD: &str = "10 GB";
 // Command-line argument and option names
 const DELETION_CHUNK_SIZE_OPTION: &str = "deletion-chunk-size";
 const KEEP_OPTION: &str = "keep";
+const DRY_RUN_OPTION: &str = "dry-run";
 const THRESHOLD_OPTION: &str = "threshold";
 
 // Size threshold argument, absolute or relative to filesystem size
@@ -109,6 +110,7 @@ pub struct Settings {
     threshold: Threshold,
     keep: Option<RegexSet>,
     deletion_chunk_size: usize,
+    dry_run: bool,
 }
 
 // Set up the logger.
@@ -186,6 +188,14 @@ fn settings() -> io::Result<Settings> {
                 .number_of_values(1)
                 .help("Prevents deletion of images for which repository:tag matches <REGEX>"),
         )
+        .arg(
+            Arg::with_name(DRY_RUN_OPTION)
+                .short("n")
+                .long(DRY_RUN_OPTION)
+                .required(false)
+                .takes_value(false)
+                .help("Dry run mode, prevents deletion of any images."),
+        )
         .arg(
             Arg::with_name(DELETION_CHUNK_SIZE_OPTION)
                 .value_name("DELETION CHUNK SIZE")
@@ -216,6 +226,11 @@ fn settings() -> io::Result<Settings> {
         None => None,
     };
 
+    let dry_run = matches.is_present(DRY_RUN_OPTION);
+    if dry_run {
+        info!("Dry-run mode enabled, will not be deleting images.");
+    }
+
     // Determine how many images to delete at once.
     let deletion_chunk_size = match matches.value_of(DELETION_CHUNK_SIZE_OPTION) {
         Some(v) => match v.parse::<usize>() {
@@ -229,6 +244,7 @@ fn settings() -> io::Result<Settings> {
         threshold,
         keep,
         deletion_chunk_size,
+        dry_run,
     })
 }