From edd9ae8ebd02e853c48d76ac25ab99fef8e5e7a0 Mon Sep 17 00:00:00 2001
From: Jan Wielemaker <jan@swi-prolog.org>
Date: Sun, 6 Oct 2024 20:44:54 +0200
Subject: [PATCH] ADDED: PL_get_stream(): SIO_TRYLOCK flag to allow failure if
 the stream is locked.

---
 man/streams.doc     |  3 +++
 src/os/SWI-Stream.h |  2 ++
 src/os/pl-file.c    | 14 ++++++++++++++
 3 files changed, 19 insertions(+)

diff --git a/man/streams.doc b/man/streams.doc
index 0ad65f3f41..8139cc7cdd 100644
--- a/man/streams.doc
+++ b/man/streams.doc
@@ -61,6 +61,9 @@ flags:
 	Get an \jargon{output stream}.  See \const{SIO_INPUT} for
         details.  If neither \const{SIO_OUTPUT} nor \const{SIO_INPUT}
 	is given \arg{t} may not be a \jargon{pair}.
+    \definition{\const{SIO_TRYLOCK}}
+	Return \const{FALSE} if the stream cannot be locked
+        immediately.  No error is generated.
     \definition{\const{SIO_NOERROR}}
 	If the function fails no exception is produced.
     \end{description}
diff --git a/src/os/SWI-Stream.h b/src/os/SWI-Stream.h
index 23abc65784..7733264bab 100644
--- a/src/os/SWI-Stream.h
+++ b/src/os/SWI-Stream.h
@@ -283,6 +283,8 @@ typedef struct io_stream
 #define SIO_BOM		SmakeFlag(31)	/* BOM was detected/written */
 #define SIO_REPPLU	SmakeFlag(32)	/* Bad char --> Prolog \uXXXX */
 
+#define SIO_TRYLOCK	SIO_CLOSING     /* Used by PL_get_stream() */
+
 #define	SIO_SEEK_SET	0	/* From beginning of file.  */
 #define	SIO_SEEK_CUR	1	/* From current position.  */
 #define	SIO_SEEK_END	2	/* From end of file.  */
diff --git a/src/os/pl-file.c b/src/os/pl-file.c
index 79ea8ccf11..7d2162a287 100644
--- a/src/os/pl-file.c
+++ b/src/os/pl-file.c
@@ -780,6 +780,7 @@ static PL_blob_t stream_blob =
 #define SH_OUTPUT   0x08		/* We want an output stream */
 #define SH_INPUT    0x10		/* We want an input stream */
 #define SH_NOPAIR   0x20		/* Do not allow for a pair */
+#define SH_TRYLOCK  0x40		/* Fail if we cannot lock */
 
 #define get_stream_handle(a, sp, flags) LDFUNC(get_stream_handle, a, sp, flags)
 static bool
@@ -828,6 +829,12 @@ get_stream_handle(DECL_LD atom_t a, IOSTREAM **sp, int flags)
     { assert( s->magic == SIO_MAGIC || s->magic == SIO_CMAGIC );
       *sp = s;
       return true;
+    } else if ( flags & SH_TRYLOCK )
+    { if ( (s=tryGetStream(s)) )
+      { *sp = s;
+	return true;
+      } else
+	return false;			/* exception */
     } else if ( (s=getStream(s)) )
     { *sp = s;
       return true;
@@ -859,6 +866,12 @@ get_stream_handle(DECL_LD atom_t a, IOSTREAM **sp, int flags)
 	  { *sp = stream;
 	    return true;
 	  }
+	} else if ( flags & SH_TRYLOCK )
+	{ if ( (s=tryGetStream(stream)) )
+	  { *sp = s;
+	    return true;
+	  } else
+	    return false;		/* exception? */
 	} else if ( (*sp = getStream(stream)) )
 	  return true;
 	goto noent;
@@ -921,6 +934,7 @@ PL_get_stream_from_blob(atom_t a, IOSTREAM **s, int flags)
 
   if ( flags&SIO_INPUT   ) myflags |= SH_INPUT;
   if ( flags&SIO_OUTPUT  ) myflags |= SH_OUTPUT;
+  if ( flags&SIO_TRYLOCK ) myflags |= SH_TRYLOCK;
   if ( flags&SIO_NOERROR ) myflags &= ~SH_ERRORS;
   if ( !(flags&(SIO_INPUT|SIO_OUTPUT)) )
     myflags |= SH_NOPAIR;