refactor: centralize infallible available_parallelism fn. Use bette…

…r channel size limit in arrow read

crates/iceberg/src/arrow/reader.rs

@@ -46,6 +46,7 @@ use crate::io::{FileIO, FileMetadata, FileRead};
 use crate::runtime::spawn;
 use crate::scan::{ArrowRecordBatchStream, FileScanTask, FileScanTaskStream};
 use crate::spec::{Datum, Schema};
+use crate::utils::available_parallelism;
 use crate::{Error, ErrorKind};
 
 /// Builder to create ArrowReader
@@ -58,9 +59,7 @@ pub struct ArrowReaderBuilder {
 impl ArrowReaderBuilder {
     /// Create a new ArrowReaderBuilder
     pub(crate) fn new(file_io: FileIO) -> Self {
-        let num_cpus = std::thread::available_parallelism()
-            .expect("failed to get number of CPUs")
-            .get();
+        let num_cpus = available_parallelism().get();
 
         ArrowReaderBuilder {
             batch_size: None,
@@ -109,16 +108,16 @@ impl ArrowReader {
     pub fn read(self, tasks: FileScanTaskStream) -> Result<ArrowRecordBatchStream> {
         let file_io = self.file_io.clone();
         let batch_size = self.batch_size;
-        let max_concurrent_fetching_datafiles = self.concurrency_limit_data_files;
+        let concurrency_limit_data_files = self.concurrency_limit_data_files;
 
-        let (tx, rx) = channel(10);
+        let (tx, rx) = channel(concurrency_limit_data_files);
         let mut channel_for_error = tx.clone();
 
         spawn(async move {
             let result = tasks
                 .map(|task| Ok((task, file_io.clone(), tx.clone())))
                 .try_for_each_concurrent(
-                    max_concurrent_fetching_datafiles,
+                    concurrency_limit_data_files,
                     |(file_scan_task, file_io, tx)| async move {
                         match file_scan_task {
                             Ok(task) => {

crates/iceberg/src/lib.rs

@@ -47,4 +47,5 @@ pub mod transform;
 mod runtime;
 
 pub mod arrow;
+mod utils;
 pub mod writer;

crates/iceberg/src/scan.rs

@@ -39,6 +39,7 @@ use crate::spec::{
     SchemaRef, SnapshotRef, TableMetadataRef,
 };
 use crate::table::Table;
+use crate::utils::available_parallelism;
 use crate::{Error, ErrorKind, Result};
 
 /// A stream of [`FileScanTask`].
@@ -62,9 +63,7 @@ pub struct TableScanBuilder<'a> {
 
 impl<'a> TableScanBuilder<'a> {
     pub(crate) fn new(table: &'a Table) -> Self {
-        let num_cpus = std::thread::available_parallelism()
-            .expect("failed to get number of CPUs")
-            .get();
+        let num_cpus = available_parallelism().get();
 
         Self {
             table,

crates/iceberg/src/utils.rs

@@ -0,0 +1,25 @@
+use std::num::NonZero;
+
+// Use a default value of 1 as the safest option.
+// See https://doc.rust-lang.org/std/thread/fn.available_parallelism.html#limitations
+// for more details.
+const DEFAULT_PARALLELISM: usize = 1;
+
+/// Uses [`std::thread::available_parallelism`] in order to
+/// retrieve an estimate of the default amount of parallelism
+/// that should be used. Note that [`std::thread::available_parallelism`]
+/// returns a `Result` as it can fail, so here we use
+/// a default value instead.
+/// Note: we don't use a OnceCell or LazyCell here as there
+/// are circumstances where the level of available
+/// parallelism can change during the lifetime of an executing
+/// process, but this should not be called in a hot loop.
+pub(crate) fn available_parallelism() -> NonZero<usize> {
+    std::thread::available_parallelism().unwrap_or_else(|_err| {
+        // Failed to get the level of parallelism.
+        // TODO: log/trace when this fallback occurs.
+
+        // Using a default value.
+        NonZero::new(DEFAULT_PARALLELISM).unwrap()
+    })
+}