pubsys: default AwsConfig so it's not required

Have serde default `aws.region` so the user does not have to have an empty
`aws.region` table if they have no region-specific configuration.

Have serde default `aws.regions` in case the user only wants to specify regions
on the command line with `PUBLISH_REGIONS`.

Derive Default on AwsConfig now that all sections of `aws` in Infra.toml are
optional, so the user doesn't need an empty `[aws]` section if they intend to
use default credential mechanisms and specify `PUBLISH_REGIONS`.

Co-authored-by: Zac Mrowicki <mrowicki@amazon.com>
Co-authored-by: Tom Kirchner <tjk@amazon.com>

tools/pubsys/src/aws/ami/mod.rs

@@ -71,9 +71,7 @@ pub(crate) async fn run(args: &Args, ami_args: &AmiArgs) -> Result<()> {
     let infra_config = InfraConfig::from_path(&args.infra_config_path).context(error::Config)?;
     trace!("Parsed infra config: {:?}", infra_config);
 
-    let aws = infra_config.aws.context(error::MissingConfig {
-        missing: "aws section",
-    })?;
+    let aws = infra_config.aws.unwrap_or_else(|| Default::default());
 
     // If the user gave an override list of regions, use that, otherwise use what's in the config.
     let mut regions = if !ami_args.regions.is_empty() {

tools/pubsys/src/config.rs

@@ -34,11 +34,13 @@ impl InfraConfig {
 }
 
 /// AWS-specific infrastructure configuration
-#[derive(Debug, Deserialize)]
+#[derive(Debug, Default, Deserialize)]
 pub(crate) struct AwsConfig {
+    #[serde(default)]
     pub(crate) regions: VecDeque<String>,
     pub(crate) role: Option<String>,
     pub(crate) profile: Option<String>,
+    #[serde(default)]
     pub(crate) region: HashMap<String, AwsRegionConfig>,
 }