mosaic() does not write files when an sprc has 1 layer

[mikemahoney218]

As of current HEAD, mosaic() does not write files when filename is present if an sprc has 1 layer.

library(terra)
#> terra 1.7.45
x <- rast(xmin=-110, xmax=-60, ymin=40, ymax=70, res=1, vals=1)
y <- rast(xmin=-95, xmax=-45, ymax=60, ymin=30, res=1, vals=2)
z <- rast(xmin=-80, xmax=-30, ymax=50, ymin=20, res=1, vals=3)

# If the sprc has multiple rasters, this works:
rsrc <- sprc(list(x, y, z))
out_file <- tempfile(fileext = ".tif")
m <- mosaic(rsrc, filename = out_file)
file.exists(out_file)
#> [1] TRUE

# If the sprc only has one raster, this fails to write a file:
rsrc <- sprc(list(x))
out_file <- tempfile(fileext = ".tif")
m <- mosaic(rsrc, filename = out_file)
file.exists(out_file)
#> [1] FALSE

^{Created on 2023-09-05 with reprex v2.0.2}

Session info

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I know this is a weird use for the sprc class, but my use case is in a package that is downloading some unknown-to-me number of tiles and uses terra to combine them into a single output image. The package always routes the downloaded tiles into an sprc, no matter how many of them exist, and as such currently fails when there is only a single tile.

[rhijmans]

Thank you for reporting this. It now works as expected.