diff --git a/.circleci/config.yml b/.circleci/config.yml
index bc94d6fd5440cf5a5ecee94c60329a3c64058437..eebd831010bc67f5e65a5668aae1bbe80a2e2929 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -241,6 +241,16 @@ workflows:
         py-version: "39"
         tahoe-lafs-source: "tahoe-lafs"
 
+    - "linux-tests":
+        name: "Linux tests python 3.9"
+        py-version: "39"
+        # This is usually not master@HEAD because it is still pinned to a
+        # certain revision.  The intent is to update it frequently and
+        # discover fixable incompatibilities in small groups and unfixable
+        # incompatibilities early enough to prevent them from going into a
+        # release.
+        tahoe-lafs-source: "tahoe-lafs-master"
+
     # https://circleci.com/docs/2.0/testing-ios/#supported-xcode-versions
     - "macos-tests":
         name: "macOS tests python 3.8 xcode 11.7.0"
diff --git a/nix/sources.json b/nix/sources.json
index fee56665c6d1b33bd996147104cda9901abcf426..5e23721d3393d58299acef4677b15b68c33cef21 100644
--- a/nix/sources.json
+++ b/nix/sources.json
@@ -64,5 +64,17 @@
         "type": "tarball",
         "url": "https://github.com/tahoe-lafs/tahoe-lafs/archive/tahoe-lafs-1.17.1.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "tahoe-lafs-master": {
+        "branch": "master",
+        "description": "The Tahoe-LAFS decentralized secure filesystem.",
+        "homepage": "https://tahoe-lafs.org/",
+        "owner": "tahoe-lafs",
+        "repo": "tahoe-lafs",
+        "rev": "766a53b5208c03c45ca0a98e97eee76870276aa1",
+        "sha256": "16mvh5ypgv9a5vhjnc78jjf4v5yzyyz19af2vixrc4zqb0ikxrxa",
+        "type": "tarball",
+        "url": "https://github.com/tahoe-lafs/tahoe-lafs/archive/766a53b5208c03c45ca0a98e97eee76870276aa1.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }