fix(nu-parser): do not update plugin.nu file on nu startup (#10007)

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# Description

I've been investigating the [issue
mentioned](https://github.com/nushell/nushell/pull/9976#issuecomment-1673290467)
in my prev pr and I've found that plugin.nu file that is used to cache
plugins signatures gets overwritten on every nushell startup and that
may actually mess up with the file content if 2 or more instances of
nushell will run simultaneously.

To reproduce:
1. register at least 2 plugins in your local nushell
2. remember how many entries you have in plugin.nu with `open
$nu.plugin-path | find nu_plugin`
3. run 
    - either `cargo test` inside nushell repo
- or run smth like this `1..100 | par-each {|it| $"(random integer
1..100)ms" | into duration | sleep $in; nu -c "$nu.plugin-path"}` to
simulate parallel access. This approach is not so reliable to reproduce
as running test but still a good point that it may effect users actually
4. validate that your `plugin.nu` file was stripped

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# Solution

In this pr I've refactored the code of handling the `register` command
to minimize code duplications and make sure that overwrite of
`plugin.nu` file is happen only when user calls the command and not on
nu startup

Another option would be to use temp `plugin.nu` when running tests, but
as the issue actually can affect users I've decided to prevent
unnecessary writing at all. Although having isolated `plugin.nu` still
worth of doing

# User-Facing Changes
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helps us keep track of breaking changes. -->
It changes the behaviour actually as the call `register <plugin>
<signature>` now doesn't updates `plugin.nu` and just reads signatures
to the memory. But as I understand that kind of call with explicit
signature is meant to use only by nushell itself in the `plugin.nu` file
only. I've asked about it in
[discord](https://discordapp.com/channels/601130461678272522/615962413203718156/1140013448915325018)

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Actually, I think the way plugins are stored might be reworked to
prevent or mitigate possible issues further:
- problem with writing to file may still arise if we try to register in
parallel as several instances will write to the same file so the lock
for the file might be required
- using additional parameters to command like `register` to implement
some internal logic could be misleading to the users
- `register` call actually affects global state of nushell that sounds a
little bit inconsistent with immutability and isolation of other parts
of the nu. See issues
[1](https://github.com/nushell/nushell/issues/8581),
[2](https://github.com/nushell/nushell/issues/8960)

crates/nu-parser/src/parse_keywords.rs

@@ -3562,68 +3562,55 @@ pub fn parse_register(working_set: &mut StateWorkingSet, spans: &[Span]) -> Pipe
     let stack = Stack::new();
     let current_envs =
         nu_engine::env::env_to_strings(working_set.permanent_state, &stack).unwrap_or_default();
-    let error = match signature {
-        Some(signature) => arguments.and_then(|(path, path_span)| {
+
+    let error = arguments.and_then(|(path, path_span)| {
         let path = path.path_buf();
         // restrict plugin file name starts with `nu_plugin_`
         let valid_plugin_name = path
             .file_name()
             .map(|s| s.to_string_lossy().starts_with("nu_plugin_"));
 
-            if let Some(true) = valid_plugin_name {
-                signature.map(|signature| {
-                    let plugin_decl = PluginDeclaration::new(path, signature, shell);
-                    working_set.add_decl(Box::new(plugin_decl));
-                    working_set.mark_plugins_file_dirty();
-                })
-            } else {
-                Err(ParseError::LabeledError(
+        let Some(true) = valid_plugin_name else {
+            return Err(ParseError::LabeledError(
                 "Register plugin failed".into(),
                 "plugin name must start with nu_plugin_".into(),
                 path_span,
-                ))
-            }
-        }),
-        None => arguments.and_then(|(path, path_span)| {
-            let path = path.path_buf();
-            // restrict plugin file name starts with `nu_plugin_`
-            let valid_plugin_name = path
-                .file_name()
-                .map(|s| s.to_string_lossy().starts_with("nu_plugin_"));
+            ));
+        };
 
-            if let Some(true) = valid_plugin_name {
-                get_signature(&path, &shell, &current_envs)
-                    .map_err(|err| {
+        let signatures = signature.map_or_else(
+            || {
+                let signatures = get_signature(&path, &shell, &current_envs).map_err(|err| {
                     ParseError::LabeledError(
                         "Error getting signatures".into(),
                         err.to_string(),
                         spans[0],
                     )
-                    })
-                    .map(|signatures| {
+                });
+
+                if signatures.is_ok() {
+                    // mark plugins file as dirty only when the user is registering plugins
+                    // and not when we evaluate plugin.nu on shell startup
+                    working_set.mark_plugins_file_dirty();
+                }
+
+                signatures
+            },
+            |sig| sig.map(|sig| vec![sig]),
+        )?;
+
         for signature in signatures {
             // create plugin command declaration (need struct impl Command)
             // store declaration in working set
-                            let plugin_decl =
-                                PluginDeclaration::new(path.clone(), signature, shell.clone());
+            let plugin_decl = PluginDeclaration::new(path.clone(), signature, shell.clone());
 
             working_set.add_decl(Box::new(plugin_decl));
         }
 
-                        working_set.mark_plugins_file_dirty();
-                    })
-            } else {
-                Err(ParseError::LabeledError(
-                    "Register plugin failed".into(),
-                    "plugin name must start with nu_plugin_".into(),
-                    path_span,
-                ))
-            }
-        }),
-    }
-    .err();
+        Ok(())
+    });
 
-    if let Some(err) = error {
+    if let Err(err) = error {
         working_set.error(err);
     }
 

crates/nu-test-support/src/macros.rs

@@ -164,6 +164,8 @@ macro_rules! nu {
         command
             .env(nu_utils::locale::LOCALE_OVERRIDE_ENV_VAR, locale)
             .env(NATIVE_PATH_ENV_VAR, paths_joined)
+            // TODO: consider adding custom plugin path for tests to
+            // not interfere with user local environment
             // .arg("--skip-plugins")
             // .arg("--no-history")
             // .arg("--config-file")