Provide vectored variants of FileRead() and FileWrite().

FileReadV() and FileWriteV() adapt pg_preadv() and pg_pwritev() for
fd.c's virtual file descriptors.  The simple FileRead() and FileWrite()
functions are now implemented in terms of the vectored functions, to
avoid code duplication, and they are converted back to the corresponding
simple system calls further down (commit 15c9ac36).  Later work will
make more interesting multi-iovec calls.

The traditional behavior of reporting a "fake" ENOSPC error is
simplified.  It's now always set for non-failing writes, for the benefit
of callers that expect to log a meaningful "%m" if they determine that
the write was short.  (Perhaps we should consider getting rid of that
expectation one day.)

Reviewed-by: Heikki Linnakangas <[email protected]>
Discussion: https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com

diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index a185fb3d08cb9ff8e8047d7c32fcfa593643568a..fbffdd72f0be388e7f159e29eaed74d531fb44fd 100644 (file)
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -2110,18 +2110,18 @@ FileWriteback(File file, off_t offset, off_t nbytes, uint32 wait_event_info)
 }
 
 int
-FileRead(File file, void *buffer, size_t amount, off_t offset,
-        uint32 wait_event_info)
+FileReadV(File file, const struct iovec *iov, int iovcnt, off_t offset,
+         uint32 wait_event_info)
 {
    int         returnCode;
    Vfd        *vfdP;
 
    Assert(FileIsValid(file));
 
-   DO_DB(elog(LOG, "FileRead: %d (%s) " INT64_FORMAT " %zu %p",
+   DO_DB(elog(LOG, "FileReadV: %d (%s) " INT64_FORMAT " %d",
               file, VfdCache[file].fileName,
               (int64) offset,
-              amount, buffer));
+              iovcnt));
 
    returnCode = FileAccess(file);
    if (returnCode < 0)
@@ -2131,7 +2131,7 @@ FileRead(File file, void *buffer, size_t amount, off_t offset,
 
 retry:
    pgstat_report_wait_start(wait_event_info);
-   returnCode = pg_pread(vfdP->fd, buffer, amount, offset);
+   returnCode = pg_preadv(vfdP->fd, iov, iovcnt, offset);
    pgstat_report_wait_end();
 
    if (returnCode < 0)
@@ -2166,18 +2166,18 @@ retry:
 }
 
 int
-FileWrite(File file, const void *buffer, size_t amount, off_t offset,
-         uint32 wait_event_info)
+FileWriteV(File file, const struct iovec *iov, int iovcnt, off_t offset,
+          uint32 wait_event_info)
 {
    int         returnCode;
    Vfd        *vfdP;
 
    Assert(FileIsValid(file));
 
-   DO_DB(elog(LOG, "FileWrite: %d (%s) " INT64_FORMAT " %zu %p",
+   DO_DB(elog(LOG, "FileWriteV: %d (%s) " INT64_FORMAT " %d",
               file, VfdCache[file].fileName,
               (int64) offset,
-              amount, buffer));
+              iovcnt));
 
    returnCode = FileAccess(file);
    if (returnCode < 0)
@@ -2195,7 +2195,10 @@ FileWrite(File file, const void *buffer, size_t amount, off_t offset,
     */
    if (temp_file_limit >= 0 && (vfdP->fdstate & FD_TEMP_FILE_LIMIT))
    {
-       off_t       past_write = offset + amount;
+       off_t       past_write = offset;
+
+       for (int i = 0; i < iovcnt; ++i)
+           past_write += iov[i].iov_len;
 
        if (past_write > vfdP->fileSize)
        {
@@ -2211,23 +2214,27 @@ FileWrite(File file, const void *buffer, size_t amount, off_t offset,
    }
 
 retry:
-   errno = 0;
    pgstat_report_wait_start(wait_event_info);
-   returnCode = pg_pwrite(VfdCache[file].fd, buffer, amount, offset);
+   returnCode = pg_pwritev(vfdP->fd, iov, iovcnt, offset);
    pgstat_report_wait_end();
 
-   /* if write didn't set errno, assume problem is no disk space */
-   if (returnCode != amount && errno == 0)
-       errno = ENOSPC;
-
    if (returnCode >= 0)
    {
+       /*
+        * Some callers expect short writes to set errno, and traditionally we
+        * have assumed that they imply disk space shortage.  We don't want to
+        * waste CPU cycles adding up the total size here, so we'll just set
+        * it for all successful writes in case such a caller determines that
+        * the write was short and ereports "%m".
+        */
+       errno = ENOSPC;
+
        /*
         * Maintain fileSize and temporary_files_size if it's a temp file.
         */
        if (vfdP->fdstate & FD_TEMP_FILE_LIMIT)
        {
-           off_t       past_write = offset + amount;
+           off_t       past_write = offset + returnCode;
 
            if (past_write > vfdP->fileSize)
            {
@@ -2239,7 +2246,7 @@ retry:
    else
    {
        /*
-        * See comments in FileRead()
+        * See comments in FileReadV()
         */
 #ifdef WIN32
        DWORD       error = GetLastError();

diff --git a/src/include/storage/fd.h b/src/include/storage/fd.h
index d9d5d9da5fb9eee1c5796fa7607df8dd87c04611..b931adce5bd9b4fa244c1d1652ee792331748d7c 100644 (file)
--- a/src/include/storage/fd.h
+++ b/src/include/storage/fd.h
@@ -15,7 +15,7 @@
 /*
  * calls:
  *
- * File {Close, Read, Write, Size, Sync}
+ * File {Close, Read, ReadV, Write, WriteV, Size, Sync}
  * {Path Name Open, Allocate, Free} File
  *
  * These are NOT JUST RENAMINGS OF THE UNIX ROUTINES.
@@ -43,6 +43,8 @@
 #ifndef FD_H
 #define FD_H
 
+#include "port/pg_iovec.h"
+
 #include <dirent.h>
 #include <fcntl.h>
 
@@ -105,8 +107,8 @@ extern File PathNameOpenFilePerm(const char *fileName, int fileFlags, mode_t fil
 extern File OpenTemporaryFile(bool interXact);
 extern void FileClose(File file);
 extern int FilePrefetch(File file, off_t offset, off_t amount, uint32 wait_event_info);
-extern int FileRead(File file, void *buffer, size_t amount, off_t offset, uint32 wait_event_info);
-extern int FileWrite(File file, const void *buffer, size_t amount, off_t offset, uint32 wait_event_info);
+extern int FileReadV(File file, const struct iovec *ioc, int iovcnt, off_t offset, uint32 wait_event_info);
+extern int FileWriteV(File file, const struct iovec *ioc, int iovcnt, off_t offset, uint32 wait_event_info);
 extern int FileSync(File file, uint32 wait_event_info);
 extern int FileZero(File file, off_t offset, off_t amount, uint32 wait_event_info);
 extern int FileFallocate(File file, off_t offset, off_t amount, uint32 wait_event_info);
@@ -189,4 +191,28 @@ extern int durable_unlink(const char *fname, int elevel);
 extern void SyncDataDirectory(void);
 extern int data_sync_elevel(int elevel);
 
+static inline int
+FileRead(File file, void *buffer, size_t amount, off_t offset,
+        uint32 wait_event_info)
+{
+   struct iovec iov = {
+       .iov_base = buffer,
+       .iov_len = amount
+   };
+
+   return FileReadV(file, &iov, 1, offset, wait_event_info);
+}
+
+static inline int
+FileWrite(File file, const void *buffer, size_t amount, off_t offset,
+         uint32 wait_event_info)
+{
+   struct iovec iov = {
+       .iov_base = unconstify(void *, buffer),
+       .iov_len = amount
+   };
+
+   return FileWriteV(file, &iov, 1, offset, wait_event_info);
+}
+
 #endif                         /* FD_H */