From 4cdfdef2ff441f5db01dc989a90aecce4dc6b272 Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Mon, 21 May 2018 15:43:30 -0700
Subject: [PATCH] WIP: Add FileOpenForFd().

Author: Andres Freund
Reviewed-By:
Discussion: https://postgr.es/m/
Backpatch:
---
 src/backend/storage/file/fd.c | 68 +++++++++++++++++++++++++++++++----
 src/include/storage/fd.h      |  2 ++
 2 files changed, 64 insertions(+), 6 deletions(-)

diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index 8ae13a51ec..50a1cb930f 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -180,6 +180,7 @@ int			max_safe_fds = 32;	/* default if not changed */
 #define FD_DELETE_AT_CLOSE	(1 << 0)	/* T = delete when closed */
 #define FD_CLOSE_AT_EOXACT	(1 << 1)	/* T = close at eoXact */
 #define FD_TEMP_FILE_LIMIT	(1 << 2)	/* T = respect temp_file_limit */
+#define FD_NOT_IN_LRU		(1 << 3)	/* T = not in LRU */
 
 typedef struct vfd
 {
@@ -304,7 +305,6 @@ static void LruDelete(File file);
 static void Insert(File file);
 static int	LruInsert(File file);
 static bool ReleaseLruFile(void);
-static void ReleaseLruFiles(void);
 static File AllocateVfd(void);
 static void FreeVfd(File file);
 
@@ -1176,7 +1176,7 @@ ReleaseLruFile(void)
  * Release kernel FDs as needed to get under the max_safe_fds limit.
  * After calling this, it's OK to try to open another file.
  */
-static void
+void
 ReleaseLruFiles(void)
 {
 	while (nfile + numAllocatedDescs >= max_safe_fds)
@@ -1289,9 +1289,11 @@ FileAccess(File file)
 		 * We now know that the file is open and that it is not the last one
 		 * accessed, so we need to move it to the head of the Lru ring.
 		 */
-
-		Delete(file);
-		Insert(file);
+		if (!(VfdCache[file].fdstate & FD_NOT_IN_LRU))
+		{
+			Delete(file);
+			Insert(file);
+		}
 	}
 
 	return 0;
@@ -1414,6 +1416,56 @@ PathNameOpenFilePerm(const char *fileName, int fileFlags, mode_t fileMode)
 	return file;
 }
 
+/*
+ * Open a File for a pre-existing file descriptor.
+ *
+ * Note that these files will not be closed in an LRU basis, therefore the
+ * caller is responsible for limiting the number of open file descriptors.
+ *
+ * The passed in name is purely for informational purposes.
+ */
+File
+FileOpenForFd(int fd, const char *fileName)
+{
+	char	   *fnamecopy;
+	File		file;
+	Vfd		   *vfdP;
+
+	/*
+	 * We need a malloc'd copy of the file name; fail cleanly if no room.
+	 */
+	fnamecopy = strdup(fileName);
+	if (fnamecopy == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_OUT_OF_MEMORY),
+				 errmsg("out of memory")));
+
+	file = AllocateVfd();
+	vfdP = &VfdCache[file];
+
+	/* Close excess kernel FDs. */
+	ReleaseLruFiles();
+
+	vfdP->fd = fd;
+	++nfile;
+
+	DO_DB(elog(LOG, "FileOpenForFd: success %d/%d (%s)",
+			   file, fd, fnamecopy));
+
+	/* NB: Explicitly not inserted into LRU! */
+
+	vfdP->fileName = fnamecopy;
+	/* Saved flags are adjusted to be OK for re-opening file */
+	vfdP->fileFlags = 0;
+	vfdP->fileMode = 0;
+	vfdP->seekPos = 0;
+	vfdP->fileSize = 0;
+	vfdP->fdstate = FD_NOT_IN_LRU;
+	vfdP->resowner = NULL;
+
+	return file;
+}
+
 /*
  * Create directory 'directory'.  If necessary, create 'basedir', which must
  * be the directory above it.  This is designed for creating the top-level
@@ -1760,7 +1812,11 @@ FileClose(File file)
 		vfdP->fd = VFD_CLOSED;
 
 		/* remove the file from the lru ring */
-		Delete(file);
+		if (!(vfdP->fdstate & FD_NOT_IN_LRU))
+		{
+			vfdP->fdstate &= ~FD_NOT_IN_LRU;
+			Delete(file);
+		}
 	}
 
 	if (vfdP->fdstate & FD_TEMP_FILE_LIMIT)
diff --git a/src/include/storage/fd.h b/src/include/storage/fd.h
index 5e016d69a5..e96e8b1398 100644
--- a/src/include/storage/fd.h
+++ b/src/include/storage/fd.h
@@ -65,6 +65,7 @@ extern int	max_safe_fds;
 /* Operations on virtual Files --- equivalent to Unix kernel file ops */
 extern File PathNameOpenFile(const char *fileName, int fileFlags);
 extern File PathNameOpenFilePerm(const char *fileName, int fileFlags, mode_t fileMode);
+extern File FileOpenForFd(int fd, const char *fileName);
 extern File OpenTemporaryFile(bool interXact);
 extern void FileClose(File file);
 extern int	FilePrefetch(File file, off_t offset, int amount, uint32 wait_event_info);
@@ -127,6 +128,7 @@ extern void AtEOSubXact_Files(bool isCommit, SubTransactionId mySubid,
 				  SubTransactionId parentSubid);
 extern void RemovePgTempFiles(void);
 extern bool looks_like_temp_rel_name(const char *name);
+extern void ReleaseLruFiles(void);
 
 extern int	pg_fsync(int fd);
 extern int	pg_fsync_no_writethrough(int fd);
-- 
2.39.5