From a1469cc8f4615364e61a3e66feb70813e5d73190 Mon Sep 17 00:00:00 2001
From: Alexander Bantyev <balsoft@balsoft.ru>
Date: Tue, 5 May 2020 13:04:36 +0300
Subject: [PATCH] Wait for build users when none are available

(cherry picked from commit 880a62b08443a6baa55dab027b69bb8b1551a588)
---
 src/libstore/build.cc | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 4b345fe37..faa41c0f0 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -508,6 +508,9 @@ private:
     Path fnUserLock;
     AutoCloseFD fdUserLock;
 
+    bool findFreeUser();
+
+
     string user;
     uid_t uid;
     gid_t gid;
@@ -528,13 +531,22 @@ public:
 
 };
 
-
 Sync<PathSet> UserLock::lockedPaths_;
 
 
 UserLock::UserLock()
 {
     assert(settings.buildUsersGroup != "");
+    createDirs(settings.nixStateDir + "/userpool");
+
+    if (findFreeUser()) return;
+
+    printError("waiting for build users");
+
+    do std::this_thread::sleep_for(std::chrono::seconds(2)); while (! findFreeUser());
+}
+
+bool UserLock::findFreeUser() {
 
     /* Get the members of the build-users-group. */
     struct group * gr = getgrnam(settings.buildUsersGroup.get().c_str());
@@ -564,7 +576,6 @@ UserLock::UserLock()
             throw Error(format("the user '%1%' in the group '%2%' does not exist")
                 % i % settings.buildUsersGroup);
 
-        createDirs(settings.nixStateDir + "/userpool");
 
         fnUserLock = (format("%1%/userpool/%2%") % settings.nixStateDir % pw->pw_uid).str();
 
@@ -605,20 +616,17 @@ UserLock::UserLock()
                 supplementaryGIDs.resize(ngroups);
 #endif
 
-                return;
+                return true;
             }
 
         } catch (...) {
             lockedPaths_.lock()->erase(fnUserLock);
+            return false;
         }
     }
-
-    throw Error(format("all build users are currently in use; "
-        "consider creating additional users and adding them to the '%1%' group")
-        % settings.buildUsersGroup);
+    return false;
 }
 
-
 UserLock::~UserLock()
 {
     auto lockedPaths(lockedPaths_.lock());
@@ -626,7 +634,6 @@ UserLock::~UserLock()
     lockedPaths->erase(fnUserLock);
 }
 
-
 void UserLock::kill()
 {
     killUser(uid);