Use fetch middlewares in event-source-polifyll (re: #23015)

[alexef]

PoC to validate this would be the way forward in #23015 (scaffolder, techdocs use event-source-polyfill library that only allows headers to be passed but not the entire fetch implementation)

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
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[backstage-goalie]

Missing Changesets

The following package(s) are changed by this PR but do not have a changeset:

• @backstage/core-app-api
• @backstage/core-plugin-api
• @backstage/test-utils
• @backstage/plugin-scaffolder
• @backstage/plugin-techdocs

See CONTRIBUTING.md for more information about how to add changesets.

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage/core-app-api	packages/core-app-api	none	v1.12.5
@backstage/core-plugin-api	packages/core-plugin-api	none	v1.9.2
@backstage/test-utils	packages/test-utils	none	v1.5.5
@backstage/plugin-scaffolder	plugins/scaffolder	none	v1.20.0
@backstage/plugin-techdocs	plugins/techdocs	none	v1.10.5

[alexef]

@aramissennyeydd / @freben pulling you from the original ticket, can you have a look and see if this would be an acceptable approach?

I can implement it as an eventSourceApi as you suggested, but I wouldn't want to duplicate the header.name configuration in FetchMiddlewares.injectIdentityAuth.

Not yet sure, but I assume if the new eventSourceApi depended on the fetchApi, then it could call the new fetchApi.headers? method so the configuration is done only once.

My current code looks like this:

createApiFactory({
    api: fetchApiRef,
    deps: {
      configApi: configApiRef,
      identityApi: identityApiRef,
      discoveryApi: discoveryApiRef,
    },
    factory: ({ configApi, identityApi, discoveryApi }) => {
      return createFetchApi({
        middleware: [
          FetchMiddlewares.resolvePluginProtocol({
            discoveryApi,
          }),
          FetchMiddlewares.injectIdentityAuth({
            identityApi,
            config: configApi,
            header: {
              name: 'Backstage-Authorization',
              value: (backstageToken) => `Bearer ${backstageToken}`,
            }
          }),
        ],
      });
    },
  }),

[alexef]

this is a test

[Rugvip]

Thank you! 👍

Wanna note here as well that I think we should explore #23015 (comment) before making additions to the core APIs

[alexef]

@Rugvip please check #24861 which should replace this PR if viable.

[alexef]

closing in favor of: #24861

[github-actions]

Uffizzi Cluster pr-24820 was deleted.