fix storing filenames longer than 100bytes in tar headers

tar/src/header.rs

@@ -6,7 +6,7 @@ use std::{
     fmt::{self, Write},
     fs::{self, Metadata},
     ops::Deref,
-    os::{linux::fs::MetadataExt, unix::fs::FileTypeExt},
+    os::{linux::fs::MetadataExt, unix::fs::FileTypeExt}, path::PathBuf, io,
 };
 
 #[repr(u8)]
@@ -203,9 +203,36 @@ impl Header {
         })
     }
 
+    pub fn prefix(&self) -> Result<String, fmt::Error> {
+        let mut s = String::new();
+        for c in self.file_prefix {
+            if c != b'\0' {
+                write!(s, "{c}")?;
+            } else {
+                break;
+            }
+        }
+        Ok(s)
+    }
+
     pub fn new(filename: &str) -> Result<Self, Error> {
         let mut header = Header::default();
         let meta = fs::symlink_metadata(filename)?;
+        let (filename, prefix) = if filename.len() < 100 {
+            (filename.to_string(), None)
+        } else {
+            // Deal with file names longer than 100 bytes
+            let path = PathBuf::from(&filename);
+            let name = match path.file_name().and_then(|n| n.to_str()) {
+                Some(n) => n.to_string(),
+                None => return Err(Error::Io(io::Error::new(io::ErrorKind::Other, "Cannot get file name"))),
+            };
+            let dir = match path.parent() {
+                Some(d) => d,
+                None => return Err(Error::Io(io::Error::new(io::ErrorKind::Other, "Cannot get path prefix"))),
+            };
+            (name, Some(format!("{}", dir.display())))
+        };
 
         /* Fill in metadata */
         header.fname[..filename.len()].copy_from_slice(filename.as_bytes());
@@ -219,6 +246,9 @@ impl Header {
         header.size[..size.len()].copy_from_slice(size.as_bytes());
         let mtime = format!("{:011o}", meta.st_mtime());
         header.mtime[..mtime.len()].copy_from_slice(mtime.as_bytes());
+        if let Some(prefix) = prefix {
+            header.file_prefix[..prefix.len()].copy_from_slice(prefix.as_bytes());
+        }
 
         /* Get the file type and conditional metadata */
         header.link_indicator[0] = FileType::from(&meta) as u8;