fix(mmap): pre-allocate temp file before mmapping (#50)

Fixes: https://github.com/zkat/cacache-rs/issues/48

This avoids SIGBUS on memory write in case the temp file is sparse.
Implemented for linux only; other target_os cfg values unchanged.

Cargo.toml

@@ -36,6 +36,9 @@ tokio = { version = "1.12.0", features = [
 tokio-stream = { version = "0.1.7", features = ["io-util"], optional = true }
 walkdir = "2.3.2"
 
+[target.'cfg(target_os = "linux")'.dependencies]
+libc = { version = "0.2.144", optional = true }
+
 [dev-dependencies]
 async-attributes = { version = "1.1.2" }
 criterion = "0.4.0"
@@ -54,6 +57,6 @@ harness = false
 
 [features]
 default = ["async-std", "mmap"]
-mmap = ["memmap2"]
+mmap = ["memmap2", "libc"]
 link_to = []
 tokio-runtime = ["tokio", "tokio-stream"]

src/content/write.rs

@@ -413,21 +413,44 @@ impl AsyncWriter {
 #[cfg(feature = "mmap")]
 fn make_mmap(tmpfile: &mut NamedTempFile, size: Option<usize>) -> Result<Option<MmapMut>> {
     if let Some(size @ 0..=MAX_MMAP_SIZE) = size {
-        tmpfile
+        allocate_file(tmpfile.as_file(), size).with_context(|| {
-            .as_file_mut()
+            format!(
-            .set_len(size as u64)
+                "Failed to configure file length for temp file at {}",
-            .with_context(|| {
+                tmpfile.path().display()
-                format!(
+            )
-                    "Failed to configure file length for temp file at {}",
+        })?;
-                    tmpfile.path().display()
-                )
-            })?;
         Ok(unsafe { MmapMut::map_mut(tmpfile.as_file()).ok() })
     } else {
         Ok(None)
     }
 }
 
+#[cfg(feature = "mmap")]
+#[cfg(target_os = "linux")]
+fn allocate_file(file: &std::fs::File, size: usize) -> std::io::Result<()> {
+    use std::io::{Error, ErrorKind};
+    use std::os::fd::AsRawFd;
+
+    let fd = file.as_raw_fd();
+    match unsafe { libc::posix_fallocate64(fd, 0, size as i64) } {
+        0 => Ok(()),
+        libc::ENOSPC => Err(Error::new(
+            ErrorKind::Other, // ErrorKind::StorageFull is unstable
+            "cannot allocate file: no space left on device",
+        )),
+        err => Err(Error::new(
+            ErrorKind::Other,
+            format!("posix_fallocate64 failed with code {err}"),
+        )),
+    }
+}
+
+#[cfg(feature = "mmap")]
+#[cfg(not(target_os = "linux"))]
+fn allocate_file(file: &std::fs::File, size: usize) -> std::io::Result<()> {
+    file.set_len(size as u64)
+}
+
 #[cfg(not(feature = "mmap"))]
 fn make_mmap(_: &mut NamedTempFile, _: Option<usize>) -> Result<Option<MmapMut>> {
     Ok(None)