fix(commons): do not fail without Vars section

If the `Vars` section was not present in a file that is parsed by the
function `try_parse_from_file` then an error would be thrown.

This commit changes the behaviour of the function such that if it is
missing, no error is returned and instead a log message is recorded.

* Cargo.lock: added dependency `tracing`.
* zenoh-flow-commons/Cargo.toml: added dependency `tracing`.

* zenoh-flow-commons/src/utils.rs: instead of throwing an error if the
  `Vars` section is missing, log a debug message and continue the normal
  execution.

Signed-off-by: Julien Loudet <[email protected]>

zenoh-flow-commons/Cargo.toml

@@ -31,6 +31,7 @@ humantime = "2.1"
 serde = { workspace = true }
 serde_json = { workspace = true }
 serde_yaml = { workspace = true }
+tracing = { workspace = true }
 uuid = { workspace = true }
 zenoh-keyexpr = { workspace = true }
 zenoh-protocol = { workspace = true }

zenoh-flow-commons/src/utils.rs

@@ -23,13 +23,15 @@ use serde::Deserialize;
 
 use crate::{IMergeOverwrite, Result, Vars};
 
-/// Given the [Path] of a file, return the function we should call to deserialize an instance of `N`.
+/// Given the [Path] of a file, return the function we should call to deserialize an instance of
+/// `N`.
 ///
 /// This function will look at the extension of the [Path] to decide on a deserializer.
 ///
 /// # Errors
 ///
-/// This function will fail if the extension of the [Path] is not supported. For now, the only supported extensions are:
+/// This function will fail if the extension of the [Path] is not supported. For now, the only
+/// supported extensions are:
 /// - ".yml"
 /// - ".yaml"
 /// - ".json"
@@ -63,25 +65,27 @@ Currently supported file extensions are:
     }
 }
 
-/// Attempts to parse an instance of `N` from the content of the file located at `path`, overwriting (or complementing)
-/// the [Vars] declared in said file with the provided `vars`.
+/// Attempts to parse an instance of `N` from the content of the file located at `path`, overwriting
+/// (or complementing) the [Vars] declared in said file with the provided `vars`.
 ///
-/// This function is notably used to parse a data flow descriptor. Two file types are supported, identified by their
-/// extension:
+/// This function is notably used to parse a data flow descriptor. Two file types are supported,
+/// identified by their extension:
 /// - JSON (`.json` file extension)
 /// - YAML (`.yaml` or `.yml` extensions)
 ///
-/// This function does not impose writing *all* descriptor file(s), within the same data flow, in the same format.
+/// This function does not impose writing *all* descriptor file(s), within the same data flow, in
+/// the same format.
 ///
 /// # Errors
 ///
 /// The parsing can fail for several reasons (listed in sequential order):
 /// - the OS failed to [canonicalize](std::fs::canonicalize()) the path of the file,
 /// - the OS failed to open (in read mode) the file,
-/// - the extension of the file is not supported by Zenoh-Flow (i.e. it's neither a YAML file or a JSON file),
+/// - the extension of the file is not supported by Zenoh-Flow (i.e. it's neither a YAML file or a
+///   JSON file),
 /// - parsing the [Vars] section failed (if there is one),
-/// - expanding the variables located in the [Vars] section failed (if there are any) --- see the documentation
-///   [handlebars] for a more complete list of reasons,
+/// - expanding the variables located in the [Vars] section failed (if there are any) --- see the
+///   documentation [handlebars] for a more complete list of reasons,
 /// - parsing an instance of `N` failed.
 pub fn try_parse_from_file<N>(path: impl AsRef<Path>, vars: Vars) -> Result<(N, Vars)>
 where
@@ -101,18 +105,29 @@ where
             path_buf.display()
         ))?;
 
-    let merged_vars = vars.merge_overwrite(
-        deserializer::<Vars>(&path_buf)?(&buf).context("Failed to deserialize Vars")?,
-    );
+    let mut merged_vars = vars;
+    match deserializer::<Vars>(&path_buf)?(&buf) {
+        Ok(parsed_vars) => {
+            merged_vars = merged_vars.merge_overwrite(parsed_vars);
+        }
+        Err(e) => {
+            // NOTE: Maybe the deserialisation fails because there is no `Vars` section. This is not
+            // necessarily an issue, hence the level "debug" for the log.
+            tracing::debug!("Could not parse Vars");
+            tracing::trace!("{e:?}");
+            tracing::trace!("Maybe the above error is normal as there is no `Vars` section?")
+        }
+    }
 
     let mut handlebars = Handlebars::new();
     handlebars.set_strict_mode(true);
 
     let rendered_descriptor = handlebars
-        // NOTE: We have to dereference `merged_vars` (this: `&(*merged_vars)`) and pass the contained `HashMap` such
-        // that `handlebars` can correctly manipulate it.
+        // NOTE: We have to dereference `merged_vars` (this: `&(*merged_vars)`) and pass the
+        // contained `HashMap` such that `handlebars` can correctly manipulate it.
         //
-        // We have to have this indirection in the structure such that `serde` can correctly deserialise the descriptor.
+        // We have to have this indirection in the structure such that `serde` can correctly
+        // deserialise the descriptor.
         .render_template(buf.as_str(), &(*merged_vars))
         .context("Failed to expand descriptor")?;