[PATCH v5 proxmox-backup 05/16] datastore: support writing fidx files of unknown size

[Robert Obkircher <r.obkircher@proxmox.com>]

Use mremap and ftruncate to support growable FixedIndexWriters. Grow
exponentially from a small initial index size and truncate excessive
capacity after encountering a non-full block or on close.

Signed-off-by: Robert Obkircher <r.obkircher@proxmox.com>
---
 pbs-datastore/src/datastore.rs   |   2 +-
 pbs-datastore/src/fixed_index.rs | 127 ++++++++++++++++++++++++++++++-
 src/api2/backup/mod.rs           |   4 +-
 3 files changed, 127 insertions(+), 6 deletions(-)

diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
index b023b0d3..694a5dd0 100644
--- a/pbs-datastore/src/datastore.rs
+++ b/pbs-datastore/src/datastore.rs
@@ -695,7 +695,7 @@ impl DataStore {
     pub fn create_fixed_writer<P: AsRef<Path>>(
         &self,
         filename: P,
-        size: usize,
+        size: Option<usize>,
         chunk_size: usize,
     ) -> Result<FixedIndexWriter, Error> {
         let full_path = self.inner.chunk_store.relative_path(filename.as_ref());
diff --git a/pbs-datastore/src/fixed_index.rs b/pbs-datastore/src/fixed_index.rs
index becbe3e3..bcb0cf94 100644
--- a/pbs-datastore/src/fixed_index.rs
+++ b/pbs-datastore/src/fixed_index.rs
@@ -217,9 +217,12 @@ pub struct FixedIndexWriter {
     chunk_size: usize,
     size: usize,
     index_length: usize,
+    index_capacity: usize,
     index: *mut u8,
     pub uuid: [u8; 16],
     pub ctime: i64,
+    growable_size: bool,
+    write_size_on_close: bool,
 }
 
 // `index` is mmap()ed which cannot be thread-local so should be sendable
@@ -235,11 +238,21 @@ impl Drop for FixedIndexWriter {
 }
 
 impl FixedIndexWriter {
+    /// The initial capacity, if the total size is unknown.
+    ///
+    /// This capacity takes up the same amount of space as the header
+    /// and can refer to 128 Blocks * 4 MiB/Block = 512 MiB of content.
+    ///
+    /// On systems with 4 KiB page size this value ensures that the
+    /// mapped length is a multiple of the page size, but this is not
+    /// strictly necessary.
+    const INITIAL_CAPACITY: usize = 4096 / 32;
+
     #[allow(clippy::cast_ptr_alignment)]
     // Requires obtaining a shared chunk store lock beforehand
     pub fn create(
         full_path: impl Into<PathBuf>,
-        size: usize,
+        known_size: Option<usize>,
         chunk_size: usize,
     ) -> Result<Self, Error> {
         let full_path = full_path.into();
@@ -261,6 +274,7 @@ impl FixedIndexWriter {
         }
 
         let ctime = proxmox_time::epoch_i64();
+        let size = known_size.unwrap_or(0);
 
         let uuid = Uuid::generate();
 
@@ -277,8 +291,12 @@ impl FixedIndexWriter {
 
         file.write_all(&buffer)?;
 
-        let index_length = size.div_ceil(chunk_size);
-        let index_size = index_length * 32;
+        let (index_length, index_capacity) = known_size
+            .map(|s| s.div_ceil(chunk_size))
+            .map(|len| (len, len))
+            .unwrap_or((0, Self::INITIAL_CAPACITY));
+
+        let index_size = index_capacity * 32;
         nix::unistd::ftruncate(&file, (header_size + index_size) as i64)?;
 
         let data = unsafe {
@@ -302,12 +320,90 @@ impl FixedIndexWriter {
             chunk_size,
             size,
             index_length,
+            index_capacity,
             index: data,
             ctime,
             uuid: *uuid.as_bytes(),
+            growable_size: known_size.is_none(),
+            write_size_on_close: known_size.is_none(),
+        })
+    }
+
+    /// If this returns an error, the sizes may be out of sync,
+    /// which is especially bad if the capacity was reduced.
+    fn set_index_capacity(&mut self, new_capacity: usize) -> Result<(), Error> {
+        if new_capacity == self.index_capacity {
+            return Ok(());
+        }
+        let old_index_size = self.index_capacity * 32;
+        let new_index_size = new_capacity * 32;
+        let new_file_size = (size_of::<FixedIndexHeader>() + new_index_size) as i64;
+
+        let index_addr = NonNull::new(self.index as *mut std::ffi::c_void).ok_or_else(|| {
+            format_err!("Can't resize FixedIndexWriter index because the index pointer is null.")
+        })?;
+
+        nix::unistd::ftruncate(&self.file, new_file_size)?;
+
+        let new_index = unsafe {
+            nix::sys::mman::mremap(
+                index_addr,
+                old_index_size,
+                new_index_size,
+                nix::sys::mman::MRemapFlags::MREMAP_MAYMOVE,
+                None,
+            )
+        }?;
+
+        self.index = new_index.as_ptr().cast::<u8>();
+        self.index_capacity = new_capacity;
+        Ok(())
+    }
+
+    /// Unmapping ensures future add and close operations fail.
+    fn set_index_capacity_or_unmap(&mut self, new_capacity: usize) -> Result<(), Error> {
+        self.set_index_capacity(new_capacity).map_err(|e| {
+            let unmap_result = self.unmap();
+            let message = format!(
+                "failed to resize index capacity from {} to {new_capacity} with backing file: {:?}",
+                self.index_capacity, self.tmp_filename
+            );
+            assert!(self.index.is_null(), "{message} {unmap_result:?}");
+            e.context(message)
         })
     }
 
+    /// Increase the content size to be at least `requested_size` and
+    /// ensure there is enough capacity.
+    ///
+    /// Only writers that were created without a known size can grow.
+    /// The size also becomes fixed as soon as it is no longer divisible
+    /// by the block size, to ensure that only the last block can be
+    /// smaller.
+    pub fn grow_to_size(&mut self, requested_size: usize) -> Result<(), Error> {
+        if self.size < requested_size {
+            if !self.growable_size {
+                bail!("refusing to resize from {} to {requested_size}", self.size);
+            }
+            let new_len = requested_size.div_ceil(self.chunk_size);
+            if new_len * self.chunk_size != requested_size {
+                // not a full chunk, so this must be the last one
+                self.growable_size = false;
+                self.set_index_capacity_or_unmap(new_len)?;
+            } else if new_len > self.index_capacity {
+                let new_capacity = new_len
+                    .checked_next_power_of_two()
+                    .ok_or_else(|| format_err!("capacity overflow"))?;
+                self.set_index_capacity_or_unmap(new_capacity)?;
+            }
+            assert!(new_len <= self.index_capacity);
+            self.index_length = new_len;
+            self.size = requested_size;
+        }
+        Ok(())
+    }
+
+    /// The current length of the index.
     pub fn index_length(&self) -> usize {
         self.index_length
     }
@@ -317,7 +413,7 @@ impl FixedIndexWriter {
             return Ok(());
         };
 
-        let index_size = self.index_length * 32;
+        let index_size = self.index_capacity * 32;
 
         if let Err(err) = unsafe { nix::sys::mman::munmap(index, index_size) } {
             bail!("unmap file {:?} failed - {}", self.tmp_filename, err);
@@ -339,9 +435,24 @@ impl FixedIndexWriter {
 
         self.unmap()?;
 
+        if self.index_length == 0 {
+            bail!("refusing to close empty fidx file {:?}", self.tmp_filename);
+        } else if self.index_length < self.index_capacity {
+            let file_size = size_of::<FixedIndexHeader>() + index_size;
+            nix::unistd::ftruncate(&self.file, file_size as i64)?;
+            self.index_capacity = self.index_length;
+        }
+
         let csum_offset = std::mem::offset_of!(FixedIndexHeader, index_csum);
         self.file.seek(SeekFrom::Start(csum_offset as u64))?;
         self.file.write_all(&index_csum)?;
+
+        if self.write_size_on_close {
+            let size_offset = std::mem::offset_of!(FixedIndexHeader, size);
+            self.file.seek(SeekFrom::Start(size_offset as u64))?;
+            self.file.write_all(&(self.size as u64).to_le_bytes())?;
+        }
+
         self.file.flush()?;
 
         if let Err(err) = std::fs::rename(&self.tmp_filename, &self.filename) {
@@ -404,6 +515,14 @@ impl FixedIndexWriter {
     }
 
     pub fn clone_data_from(&mut self, reader: &FixedIndexReader) -> Result<(), Error> {
+        if self.growable_size {
+            bail!("reusing the index is only supported with known input size");
+        }
+
+        if self.chunk_size != reader.chunk_size {
+            bail!("can't reuse file with different chunk size");
+        }
+
         if self.index_length != reader.index_count() {
             bail!("clone_data_from failed - index sizes not equal");
         }
diff --git a/src/api2/backup/mod.rs b/src/api2/backup/mod.rs
index 3e6b7a95..c625a2dc 100644
--- a/src/api2/backup/mod.rs
+++ b/src/api2/backup/mod.rs
@@ -528,7 +528,9 @@ fn create_fixed_index(
         reader = Some(index);
     }
 
-    let mut writer = env.datastore.create_fixed_writer(&path, size, chunk_size)?;
+    let mut writer = env
+        .datastore
+        .create_fixed_writer(&path, Some(size), chunk_size)?;
 
     if let Some(reader) = reader {
         writer.clone_data_from(&reader)?;
-- 
2.47.3