Index: src/checkin.c
==================================================================
--- src/checkin.c
+++ src/checkin.c
@@ -2161,11 +2161,15 @@
 
   /*
   ** Autosync if autosync is enabled and this is not a private check-in.
   */
   if( !g.markPrivate ){
-    if( autosync_loop(SYNC_PULL, db_get_int("autosync-tries", 1), 1) ){
+    int syncFlags = SYNC_PULL;
+    if( vid!=0 && !allowFork && !forceFlag ){
+      syncFlags |= SYNC_CKIN_LOCK;
+    }
+    if( autosync_loop(syncFlags, db_get_int("autosync-tries", 1), 1) ){
       fossil_exit(1);
     }
   }
 
   /* Require confirmation to continue with the check-in if there is
@@ -2278,11 +2282,11 @@
           "See https://fossil-scm.org/fossil/doc/trunk/www/branching.wiki#branching");
       }
       sCiInfo.zBranch = db_get("main-branch", "trunk");
     }
   }else if( sCiInfo.zBranch==0 && allowFork==0 && forceFlag==0
-    && g.markPrivate==0 && !is_a_leaf(vid)
+    && g.markPrivate==0 && (g.ckinLockFail || !is_a_leaf(vid))
   ){
     /* Can't avoid duplicating this string because some C compilers
     ** refuse to see static const char zErr[] = "... as "constant"
     ** enough for a printf() style format string.  (e.g. Clang 10)
     */

Index: src/main.c
==================================================================
--- src/main.c
+++ src/main.c
@@ -190,10 +190,11 @@
   FILE *httpOut;          /* Send HTTP output here */
   int xlinkClusterOnly;   /* Set when cloning.  Only process clusters */
   int fTimeFormat;        /* 1 for UTC.  2 for localtime.  0 not yet selected */
   int *aCommitFile;       /* Array of files to be committed */
   int markPrivate;        /* All new artifacts are private if true */
+  int ckinLockFail;       /* Check-in lock failure received from server */
   int clockSkewSeen;      /* True if clocks on client and server out of sync */
   int wikiFlags;          /* Wiki conversion flags applied to %W */
   char isHTTP;            /* True if server/CGI modes, else assume CLI. */
   char javascriptHyperlink; /* If true, set href= using script, not HTML */
   Blob httpHeader;        /* Complete text of the HTTP request header */

Index: src/xfer.c
==================================================================
--- src/xfer.c
+++ src/xfer.c
@@ -1545,10 +1545,62 @@
             send_unversioned_catalog(&xfer);
           }
         }
         uvCatalogSent = 1;
       }
+
+      /*   pragma ci-lock CHECKIN-HASH CLIENT-ID
+      **
+      ** The client wants to make non-branch commit against the check-in
+      ** identified by CHECKIN-HASH.  The server will remember this and
+      ** subsequent ci-lock request from different clients will generate
+      ** a ci-lock-fail pragma in the reply.
+      */
+      if( blob_eq(&xfer.aToken[1], "ci-lock")
+       && xfer.nToken==4
+       && blob_is_hname(&xfer.aToken[2])
+      ){
+        Stmt q;
+        sqlite3_int64 iNow = time(0);
+        int seenFault = 0;
+        db_prepare(&q,
+          "SELECT json_extract(value,'$.login'),"
+          "       mtime,"
+          "       json_extract(value,'$.clientid'),"
+          "       (SELECT rid FROM blob WHERE uuid=substr(name,9)),"
+          "       name"
+          " FROM config WHERE name GLOB 'ci-lock-*'"
+        );
+        while( db_step(&q)==SQLITE_ROW ){
+          int x = db_column_int(&q,3);
+          const char *zName = db_column_text(&q,4);
+          if( db_column_int64(&q,1)<iNow-3600*24 || !is_a_leaf(x) ){
+            /* check-in locks expire after 24 hours, or when the check-in
+            ** is no longer a leaf */
+            db_multi_exec("DELETE FROM config WHERE name=%Q", zName);
+            continue;
+          }
+          if( fossil_strcmp(zName+8, blob_str(&xfer.aToken[2]))==0 ){
+            const char *zClientId = db_column_text(&q, 2);
+            const char *zLogin = db_column_text(&q,0);
+            sqlite3_int64 mtime = db_column_int64(&q, 1);
+            if( fossil_strcmp(zClientId, blob_str(&xfer.aToken[3]))!=0 ){
+              @ pragma ci-lock-fail %F(zLogin) %lld(mtime)
+            }
+            seenFault = 1;
+          }
+        }
+        db_finalize(&q);
+        if( !seenFault ){
+          db_multi_exec(
+            "REPLACE INTO config(name,value,mtime)"
+            "VALUES('ci-lock-%q',json_object('login',%Q,'clientid',%Q),now())",
+            blob_str(&xfer.aToken[2]), g.zLogin,
+            blob_str(&xfer.aToken[3])
+          );
+        }
+      }
     }else
 
     /* Unknown message
     */
     {
@@ -1655,10 +1707,11 @@
 #define SYNC_UV_REVERT      0x0080    /* Copy server unversioned to client */
 #define SYNC_FROMPARENT     0x0100    /* Pull from the parent project */
 #define SYNC_UV_TRACE       0x0200    /* Describe UV activities */
 #define SYNC_UV_DRYRUN      0x0400    /* Do not actually exchange files */
 #define SYNC_IFABLE         0x0800    /* Inability to sync is not fatal */
+#define SYNC_CKIN_LOCK      0x1000    /* Lock the current check-in */
 #endif
 
 /*
 ** Floating-point absolute value
 */
@@ -1708,10 +1761,11 @@
   int uvDoPush = 0;       /* Generate uvfile messages to send to server */
   int nUvGimmeSent = 0;   /* Number of uvgimme cards sent on this cycle */
   int nUvFileRcvd = 0;    /* Number of uvfile cards received on this cycle */
   sqlite3_int64 mtime;    /* Modification time on a UV file */
   int autopushFailed = 0; /* Autopush following commit failed if true */
+  const char *zCkinLock;  /* Name of check-in to lock.  NULL for none */
 
   if( db_get_boolean("dont-push", 0) ) syncFlags &= ~SYNC_PUSH;
   if( (syncFlags & (SYNC_PUSH|SYNC_PULL|SYNC_CLONE|SYNC_UNVERSIONED))==0
      && configRcvMask==0 && configSendMask==0 ) return 0;
   if( syncFlags & SYNC_FROMPARENT ){
@@ -1749,10 +1803,18 @@
 
   /* Send the send-private pragma if we are trying to sync private data */
   if( syncFlags & SYNC_PRIVATE ){
     blob_append(&send, "pragma send-private\n", -1);
   }
+
+  /* Figure out which check-in to lock */
+  if( syncFlags & SYNC_CKIN_LOCK ){
+    int vid = db_lget_int("checkout",0);
+    zCkinLock = db_text(0, "SELECT uuid FROM blob WHERE rid=%d", vid);
+  }else{
+    zCkinLock = 0;
+  }
 
   /* When syncing unversioned files, create a TEMP table in which to store
   ** the names of files that need to be sent from client to server.
   **
   ** The initial assumption is that all unversioned files need to be sent
@@ -1911,10 +1973,22 @@
         }
         db_finalize(&uvq);
         if( rc==SQLITE_DONE ) uvDoPush = 0;
       }
     }
+
+    /* Lock the current check-out */
+    if( zCkinLock ){
+      const char *zClientId;
+      zClientId = db_lget("client-id", 0);
+      if( zClientId==0 ){
+        zClientId = db_text(0, "SELECT lower(hex(randomblob(20)))");
+        db_lset("client-id", zClientId);
+      }
+      blob_appendf(&send, "pragma ci-lock %s %s\n", zCkinLock, zClientId);
+      zCkinLock = 0;
+    }
 
     /* Append randomness to the end of the message.  This makes all
     ** messages unique so that that the login-card nonce will always
     ** be unique.
     */
@@ -2271,10 +2345,25 @@
         if( blob_eq(&xfer.aToken[1], "uv-pull-only") ){
           if( syncFlags & SYNC_UV_REVERT ) uvDoPush = 1;
         }else if( blob_eq(&xfer.aToken[1], "uv-push-ok") ){
           uvDoPush = 1;
         }
+
+        /*    pragma ci-lock-fail  USER-HOLDING-LOCK  LOCK-TIME
+        **
+        ** The server generates this message when a "pragma ci-lock"
+        ** is attempted on a check-in for which there is an existing
+        ** lock.  USER-HOLDING-LOCK is the name of the user who originated
+        ** the lock, and LOCK-TIME is the timestamp (seconds since 1970)
+        ** when the lock was taken.
+        */
+        else if( blob_eq(&xfer.aToken[1], "ci-lock-fail") && xfer.nToken==4 ){
+          char *zUser = blob_terminate(&xfer.aToken[2]);
+          defossilize(zUser);
+          fossil_print("\nParent check-in locked by %s\n", zUser);
+          g.ckinLockFail = 1;
+        }
       }else
 
       /*   error MESSAGE
       **
       ** Report an error and abandon the sync session.