Use randomly generated delimiter for multiline env variables

The previous discussion in #6 finally has a proper answer, this updates it to follow `@actions/core`: https://github.com/actions/toolkit/blob/b36e70495fbee083eb20f600eafa9091d832577d/packages/core/src/file-command.ts#L27-L47

Project.toml

@@ -6,6 +6,7 @@ version = "0.1.4"
 [deps]
 JSON = "682c06a0-de6a-54ab-a142-c8b1cf79cde6"
 Logging = "56ddb016-857b-54e1-b83d-db4d58db5568"
+UUIDs = "cf7118a7-6976-5b1a-9a39-7adc72f591a4"
 
 [compat]
 JSON = "0.21"

THIRD_PARTY_NOTICE.md

@@ -0,0 +1,18 @@
+# Third Party Notices and Information
+
+Parts of this software have been derived from other open source software.
+Find their full licence information below.
+
+- [`@actions/core`](https://github.com/actions/toolkit/tree/main/packages/core)
+
+## [`@actions/core`](https://github.com/actions/toolkit/tree/main/packages/core)
+
+> The MIT License (MIT)
+>
+>Copyright 2019 GitHub
+>
+> Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
+>
+> The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
+>
+> THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

src/GitHubActions.jl

@@ -22,6 +22,8 @@ using Logging: Logging, AbstractLogger, Debug, Info, Warn, Error
 
 using JSON: json
 
+using UUIDs
+
 const CMD_MARKER = "::"
 
 """
@@ -188,10 +190,12 @@ Set environment variable `k` to value `v`.
 function set_env(k, v)
     val = cmd_value(v)
     ENV[k] = val
-    delimiter = "EOF"
-    while occursin(delimiter, val)
-        delimiter *= "EOF"
-    end
+    delimiter = uuid4()
+
+    # Safety precaution in case UUID generation is exploitable
+    !occursin(string(delimiter), val) || (set_failed("value of environment variable must not contain the delimiter $delimiter"); exit())
+    !occursin(string(delimiter), k) || (set_failed("name of environment variable must not contain the delimiter $delimiter"); exit())
+
     add_to_file("GITHUB_ENV", join(["$k<<$delimiter", val, delimiter], "\n"))
 end