fix file size computation after pwrite (#4942)

lib/wasix/src/syscalls/wasi/fd_write.rs

@@ -414,7 +414,7 @@ pub(crate) fn fd_write_internal<M: MemorySize>(
 
         // reborrow and update the size
         if !is_stdio {
-            let offset = if is_file && should_update_cursor {
+            let curr_offset = if is_file && should_update_cursor {
                 let bytes_written = bytes_written as u64;
                 let mut fd_map = state.fs.fd_map.write().unwrap();
                 let fd_entry = wasi_try_ok_ok!(fd_map.get_mut(&fd).ok_or(Errno::Badf));
@@ -433,10 +433,17 @@ pub(crate) fn fd_write_internal<M: MemorySize>(
                 unsafe { env.get_memory_and_wasi_state_and_inodes(&ctx, 0) };
             if is_file {
                 let mut stat = fd_entry.inode.stat.write().unwrap();
-                // If we wrote before the end, the current size is still correct.
-                // Otherwise, we only got as far as the current cursor. So, the
-                // max of the two is the correct new size.
-                stat.st_size = stat.st_size.max(offset);
+                if should_update_cursor {
+                    // If we wrote before the end, the current size is still correct.
+                    // Otherwise, we only got as far as the current cursor. So, the
+                    // max of the two is the correct new size.
+                    stat.st_size = stat.st_size.max(curr_offset);
+                } else {
+                    // pwrite does not update the cursor of the file so to calculate the final
+                    // size of the file we compute where the cursor would have been if it was updated,
+                    // and get the max value between it and the current size.
+                    stat.st_size = stat.st_size.max(offset + bytes_written as u64);
+                }
             } else {
                 // Cast is valid because we don't support 128 bit systems...
                 fd_entry.inode.stat.write().unwrap().st_size += bytes_written as u64;

tests/wasix/pwrite-and-size/main.c

@@ -0,0 +1,86 @@
+#include <stdio.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/stat.h>
+
+int main()
+{
+    int fd;
+    char *data;
+    struct stat statbuf;
+
+    fd = open("/data/my_file.txt", O_CREAT | O_WRONLY, 0644);
+    if (fd == -1)
+    {
+        goto fail;
+    }
+
+    data = "ABC";
+
+    if (pwrite(fd, data, 3, 0) == -1)
+    {
+        goto fail;
+    }
+    if (fstat(fd, &statbuf) == -1)
+    {
+        goto fail;
+    }
+
+    if (statbuf.st_size != 3)
+    {
+        goto fail;
+    }
+
+    data = "D";
+    if (pwrite(fd, data, 1, 1) == -1)
+    {
+        goto fail;
+    }
+
+    if (fstat(fd, &statbuf) == -1)
+    {
+        goto fail;
+    }
+    if (statbuf.st_size != 3)
+    {
+        goto fail;
+    }
+
+    data = "XYZ";
+    if (pwrite(fd, data, 3, 3) == -1)
+    {
+        goto fail;
+    }
+    if (fstat(fd, &statbuf) == -1)
+    {
+        goto fail;
+    }
+    if (statbuf.st_size != 6)
+    {
+        goto fail;
+    }
+
+    data = "GHIJKLMJ";
+    if (pwrite(fd, data, 10, 1) == -1)
+    {
+        goto fail;
+    }
+    if (fstat(fd, &statbuf) == -1)
+    {
+        goto fail;
+    }
+
+    if (statbuf.st_size != 11)
+    {
+        goto fail;
+    }
+
+    printf("0");
+
+    return 0;
+
+fail:
+    printf("1");
+
+    return 1;
+}

tests/wasix/pwrite-and-size/run.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+$WASMER -q run main.wasm --mapdir=/data:. > output
+
+printf "0" | diff -u output - 1>/dev/null
+
+rm my_file.txt